main difference is, normal 3d objects size differs by how far it is. Skyboxes rendered different than other 3d objects. such as if you use different texture proportions it only causes tiling of that texture to achieve 256*256. not all errors crashes the game but not works correctly. I tried another sized textures and this causes only error.
Skybox should be only 256^2 for each plane. This problem also bothered me, i was trying to use hd skybox to use more open area maps but its imposible.
