Anything with a width or height of 8, 16, 32, 64, 128, 256, 512, 1024, 2048, or 4096. They can be mixed and matched. Above 1024 on a given side is not recommended unless mipmaps are used. 256x256 texture sizes have the fastest rendering speed.
"You level up the fastest and easiest if you do things at your own level and no higher or lower" - useful tip
My 2D game - release on Jun 13th; My tutorials
