Inspiration for homebrew can sometimes come from unexpected places. Recently, I was watching an educational Youtube video by Kurzgesagt – In a Nutshell and started rethinking Dungeon & Dragon’s understanding of creature sizes.

For those of you unacquainted, Kurzgesagt is an awesome Youtube channel dedicated to making informative videos on just about everything from quantum physics to banking. Their team recently released a new video discussing how size is a regulator for all living things and demonstrated it in perhaps the most intriguing (and disturbing way) possible: by throwing hypothetical animals from a building.

You can watch the video either below or by following this link. I recommend watching it as most of what I will be discussing today is based on the video’s content.

Current Approach to Creature Size

After watching the video,  I started pondering the possibility of a blue whale falling from a skyscraper as well as the mechanics dealing with creature size inside D&D.

To focus on the latter, it is interesting to me that D&D only considers creature size in terms of physical volume. At a basic level, it makes sense; a Purple Worm should take up more space than an Owlbear or a Dwarf. D&D resolves this difference  using the combat grid.

However, the system appears married to that one solution and does not play with size in any other meaningful or interesting ways. In the two sections dedicated to creature size (Monster Manual pg.6 and Dungeon Master’s Guide pg. 251), both focus exclusively on how the number of tiles on the combat grid change depending on a creature’s size. Additionally, almost all size-related rules, such as knock effects or squeezing, correspond with the combat grid. Only grappling, which uses creature size as a limitator, is an exception.

Even DMs are generally skimpy and basic with their implementation of creature size in their games. Possibly realizing how little size is played with in the actual mechanics, DMs only concern themselves with creature size when designing a combat grid or when considering architecture such as entryways, passages, and ceilings.

But clearly there is more to size than just how much space you are taking up or what doorways you can enter. While these are obvious considerations, they are merely the fundamentals to capturing life at a different size. As highlighted by the Kurzgesagt video, your size determines how you interact with the world. Your bathroom, for instance, is entirely different place to view and experience as a human than it would be if you were a house fly. This is also true in your fantasy world. For example, a pixie observes and experiences your campaign world in radically different ways than a fire giant does. For that pixie,  a forest leaf is a possible source of cover, traveling through a rainstorm is a heroic and life-threatening adventure, and a volcano is a literal ocean of lava as far as the eye can see.

Simply put, what is a hazard to one creature is not a hazard, or even a consideration, for the other.

Therefore to holistically capture a situation where a character is an odd or fantastical size, more can be done to truly illustrate this radical difference. In other words, more game mechanics can take a page from grappling and apply new advantages and disadvantages to creatures depending on their size.  To this end, I explore below some of the physics and examples mentioned in the Kurzgesagt video and apply game mechanics to them.

One Tiny Problem with Size Categories

Nevertheless, before I get into applying the physics from Kurzgesagt, I wanted to express one small caveat  Upon the release of D&D 5th Edition, Wizards of the Coast diminished the number of size categories. This was a great decision because it simplified and standardized many game mechanics that in previous editions were burdensome or just ignored. That being said, this change also means each size category contains a broader range of physical size. For instance, a hunting dog and a 8-foot tall Goliath are both Medium size. Likewise, an eagle, a cat, a mouse, and an insect are all considered to be Tiny size.

As you can imagine, this has many unintended side-effects when applying special rules (especially physics) and for this reason, adding more size categories is definitely recommended if you develop any new creature size mechanics based on the real world.

Applying Size to Fall Damage

The focal example of the Kurzgesagt video is the difference gravity has on creatures with different sizes. Smaller size, means smaller volume and a smaller risk from gravity. So tinier creatures, like a mouse or an insect from the video, take almost no injuries after falling, whereas huge creatures explode into a mound of blood and gore. When applying these real world physics to your game, these are some rules you could consider:

Reduced Fall Damage for Tiny Creatures

At the end of a fall, a Tiny sized creature takes 1d6 bludgeoning damage every 20 feet it fell to a maximum of 10d6.

Diminutive Creatures and Immunity to Fall Damage

As mentioned above, the tiny size category contains a substantial range of size as it is basically the lowest category anything can reach mechanically. Hence, to better match the video’s explanation of physics where a bug can survive being dropped from a plane, I recommend adding a Diminutive Size category.

Diminutive was originally in D&D 3.5e, and  it classified creatures that were only a few inches in height, or less than a pound in weight. Creatures like locust, mice, tree frogs, pixies, or fairies fall into this category as well as any other creatures that can easily fit in between your fingers.

Diminutive sized creatures are immune to all fall damage.

Additional Fall Damage for Huge and Gargantuan Creatures

At the end of a fall, a Huge sized creature takes 2d6 bludgeoning damage every 10 feet it fell to a maximum of 40d6 damage. A Gargantuan sized creature takes 3d6 bludgeoning damage every 10 feet it fell to a maximum of 60d6 damage.

Splatting Effects

If a Huge or larger sized creature takes fall damage and this damage reduces the creature to 0 hit points, the creature’s body violently erupts into a pool of blood, bones, and innards.

All creatures within 10 feet of a Huge creature’s body, (or 15 feet for a Gargantuan Creature) become coated in the fallen creature’s body parts and must succeed on a DC12 Strength saving throw or fall prone.

Applying Size to Water Hazards

Another example employed in the Kurzgesagt video is the dangers of water for tiny creatures. Given tiny creature’s surface area and the weight of water, tiny creatures like rats or eagles can be heavily weighed down by the water covering their body. Moreover, water can be a death sentence for diminutive creatures like insects. To match these real-world effects, these are some rules you could consider for your game:

Water Preventing Flight

If a tiny (or smaller) size creature falls into an area of water or becomes drenched in water, its fly speed becomes 0.

Dangers of Surface Tension

If a tiny size creature enters an area of water, the creature must make a DC 12 Strength saving throw in order to exit an area of water. On a failed save, the creature remains in its current space. This effect is ignored if a creature has a swim speed.

If a diminutive size (see Author’s Note above) creature enters an area of water,  the water’s surface tension grabs the creature. The creature must make a DC 15 Strength saving throw to avoid becoming restrained by the water. The creature can repeat this saving throw at the beginning of each turn, ending the effect. This effect is ignored if a creature has a swim speed.

Conclusions and the Dangers of Physics

Hopefully from these examples, you can see how game mechanics can be played with to better reflect real life. Likewise, I hope that these examples show how inspiration for possible game mechanics can come from things beyond video games or other tabletop systems.

It is important to note though that these new rules do not perfectly replicate real life or even the basics discussed by Kurzgesagt. This is by design. New game mechanics should aim to be as simple as possible in order for players and other DMs to properly utilize them. Ultimately, for the game to be easy and fun for players, game logic needs to prevail over realism.

Similarly, my rules do not play with Small or Medium sized creatures. This is because the actual difference in terms of physics for these creatures is rather negligible. In addition, playable races are these sizes and altering the rules for these two size categories might have unintended consequences on the game’s overall balance. This is why D&D makes Small and Medium sized creatures take up the same amount of space on a combat grid (even though they are dramatically different volume wise).

However, selectively applying physics can be a dangerous practice, as anyone acquainted with the Peasant Railgun can attest. Consequently, it’s understandable why D&D did not go further in developing game mechanics based on physics and why if you are going to use the rules above or make your own size-based rules, I would proceed with caution.

These size-based rules are, at best, an experiment and maybe should only be introduced in situations where they make the greatest impact. In those particular situations, whether it’s your players being shrunk magically to diminutive size or a whale being summoned from the top of a great magical spire, your game will feel not only realistic, but also flavorful.