Can forcecage fit creatures whose size category is larger than the cage?

Question

The forcecage spell says that the target(s) must fit entirely within the effect.

But Player's Handbook says that size categories aren't an indicator of actual size, just "combat presence".

So the question is, can creatures whose size category is larger than the forcecage effect fit within? If so, how much larger? Is there any indication of the "actual size" of creatures?

As a corollary, can we even say that creatures whose size category is smaller than the forcecage effect can fit within it?

In particular, dragons are an issue in one of my campaigns, as a DM with a high level Wizard PC, and dragon-specific answers (can they be forcecaged - yes, no, or something more complicated) are appreciated.

Answer 1

Your Reasoning is Correct...

... from a Rules as Written perspective:

Forcecage reads:

A prison in the shape of a cage can be up to 20 ft. on a side ... A prison in the shape of a box can be up to 10 ft. on a side ... Any creature that is completely inside the cage's area is trapped. [Emphasis mine]

The note on space (pg. 191) reads:

A creature's space is the area it effectively controls in combat, not an expression of its physical size. A typical medium creature isn't 5 feet wide. [Emphasis mine]

Therefore, since forcecage only describes the creature physically being inside the cage, and space does not represent physical size, a creature is trapped by forcecage if its body is fully within the cage, regardless of whether some of its space is outside.

To take your example (comment), a Storm Giant (MM pg. 153) is 26 ft. tall. It therefore cannot fit into a forcecage and therefore cannot be (fully) affected by the spell, regardless of whether or not its space is fully inside the cage. If the giant were to sit, or bend over, so as to reduce its height to under 20 ft., then it could fit inside, and could be trapped.

Physical size is everything, not controlled space.

Dragons

For this part of the answer, I am assuming that the measurements in the 3.5e Draconomicon apply to 5e.

The colour of the dragon is irrelevant in determining its size. The Draconomicon provides only a table of actual sizes by given size (tiny, small, medium, &c.). Although the table itself is huge (pg. 37), giving values for neck length, tail length, minimum wingspan, &c., I provide some of the more important values:

\begin{array}{c|ccc} \text{Size} & \text{Body length} & \text{Body width} & \text{Standing height} \\ \hline \text{Large (Young)} & 9ft & 5ft & 7ft \\ \text{Huge (Adult)} & 16ft & 8ft & 12ft \\ \text{Gargantuan (Ancient)} & 24ft & 10ft & 16ft \end{array}

Note that all of the values here represent not the maximum possible dimensions (i.e. tail and head completely stretched out), but the likely dimensions during combat (i.e. tail and head in).

Thus we can see that only the gargantuan ancient dragons are unable to fit inside a forcecage. However, dragons are clever creatures, and may stretch themselves out if they recognise that forcecage is being cast on them.

Answer 2

Forcecage only cares about the space (threat range, actual size, or otherwise) a creature takes up in its environment.

This is often measured on a grid. Medium creatures take up a 5 X 5 'space' even though they are not 5ft wide. This represents a Size that is 'Medium'. Large creatures take up a 10 X 10 'space' . So on an so forth.

The Force cage has one of 2 maximum dimensions: A 20 X 20 Cage with 1/2 inch spaced bars a 1/2 inch thick, or a 10 X 10 cube completely sealed and solid.

The spell cares about if a creature's size threshold (the dimensions it takes up as a result) fits within the chosen form. Any creature larger than 20 X 20 if cage is chosen or 10 X 10 if cube is chosen does not fit and is not 'completely inside the area'.

I am not at my computer and forget the relevant size categories but any creature 21 X 21 and larger would not fit.

It also bares mentioning that the measurements of Forcecage are 'on a side' or per side; which includes the top or 'ceiling' if you will. Creatures who fit within the maximum 10 X 10 or 20 X 20 may not be short enough to fit under the cage or cube, though this is conjecture because nothing stops you from having a very tall Goliath break the height measurement of a 'Large' 10 X 10 creature. This flavor does not then make that Goliath take up 5 X 5 X 10. It would still fit within a 5 X 5 forcecage. Unless your DM wants to add a height measurement to every potential creature because we feel flavor matters?

Answer 3

So the question is, can creatures whose size category is larger than the forcecage effect fit within? If so, how much larger?

I recommend just using the grid size. So based on that I would say "no, they do not fit".

However, don't forget that creatures can squeeze into smaller spaces, and that could help trapping e.g. by casting 10'x10' version on a huge dragon as it squeezes through a 10' passage, or if it voluntarily squeezes or is coerced etc.

Is there any indication of the "actual size" of creatures?

There are some indications of "actual size" in descriptions of key monsters - most notably the giants which have traditionally have a fixed hierarchy of sizes. Some giant creatures with unusually long or flat shapes have their sizes listed in other editions, but this doesn't appear to have carried over to the 5E SRD.

For dragons and other giant creatures with natural melee attacks, there is an implied extension of attacking parts with reach. With the dragon, the long reach of bite and tail attacks implies an overall body length greater than the space required to fight. This would seem to apply to other creatures with long appendages - tentacles, necks, tails etc.

As a corollary, can we even say that creatures whose size category is smaller than the forcecage effect can fit within it?

I suggest don't overthink this, you will create a new area of rulings that rely on you as DM judging which bits of which creatures are sticking out of the targeted area. And the problem with that is it is very situational. The Wizard player will feel the need to ask you if the spell might work or if the pose/shape is wrong. This might not be the case if all creatures had clear shapes/sizes you can compare to, but ultimately you will be making that up.

Instead, I could rationalise it working in a simple grid-space fashion by assuming part of the force effect is to push any sticking-out bits into the cage. After all, this is a high level spell where the intent of casting is always to physically trap a target. In addition, the spell already clearly does move creatures around from its description:

Creatures only partially within the area, or those too large to fit inside the area, are pushed away from the center of the area until they are completely outside the area.

so although it is not described as such, it's not unreasonable to assume equivalent forces pushing errant limbs/weapons etc of captured creatures inwards too.