The forcecage spell notes 'Creatures only partially within the area, or those too large to fit inside it, are pushed away from the center of the area until they are completely outside it.'
This is different from wall spells, such as wall of force, that refer to a creature's space. A creature's space is basically the size of the mini's stand, but plenty of minis are much larger than their stands (for example, the Wizkids adult red dragon is 8" tall on a 3" stand). Of course, unlike 3e (which used the same size classes and spaces, but stated that huge was 16-32' and gargantuan was 32-64') most of 5e doesn't actually tell you how big monsters are.
So... can a barred forcecage (20') trap a gargantuan creature? Or even a Huge creature that's more than 20', such as a 26' tall storm giant?
A creature’s size determines how much space the creature occupies in combat.
I would consider the RAW to be that for the purposes of using a spell like Forcecage where you're defining an area on the map, if you can surround the space a size category covers you're good even if aesthetically the creature can be considered to have a larger dimension.
The forcecage spell notes 'Creatures only partially within the area, or those too large to fit inside it, are pushed away from the center of the area until they are completely outside it.'
This is different from wall spells, such as wall of force, that refer to a creature's space. A creature's space is basically the size of the mini's stand, but plenty of minis are much larger than their stands (for example, the Wizkids adult red dragon is 8" tall on a 3" stand). Of course, unlike 3e (which used the same size classes and spaces, but stated that huge was 16-32' and gargantuan was 32-64') most of 5e doesn't actually tell you how big monsters are.
So... can a barred forcecage (20') trap a gargantuan creature? Or even a Huge creature that's more than 20', such as a 26' tall storm giant?
The PHB has a definition of size here.
Since it reads:
I would consider the RAW to be that for the purposes of using a spell like Forcecage where you're defining an area on the map, if you can surround the space a size category covers you're good even if aesthetically the creature can be considered to have a larger dimension.
I agree with ace of rogues. I’d stick with 2 dimensions. The game really starts to get confusing when the z-axis shows up.
Even the z-axis shouldn't make a difference, since I believe it's just supposed to treat sizes as a cube rather than a square.