Test map demonstrating use of textures up to 544x544. The map will run only if you embed them into the map, otherwise, you will get a TEX_LoadLump: texture lump too large to load error. Anything higher in size will result in a Mod_PointInLeaf: bad model error.
No I meant 1024. Like, they have to be shared by both sides, making the max 512x512 to be equally long. Or 768x256, etc... Basically the sum of both sides doesn't have to exceed 1024 pixels.
Out of curiosity, does 8192x32 work?