I know skyboxes like to be 256^2 and no more, but I've seen textures that are double that resolution at 512^2 and work perfectly fine. If I try any environment box over 256^2, it will give me some error and the map won't load, so I size them down.
This leads to my question, why can't an skybox be 512^2, but a normal texture can?
Also, what is the most an normal texture could be?