The maximum spritesheet size is 2048x2048, but if it can fit on a smaller power of two size (1024x1024, 512x512 etc) then it will. Since every image gets a 1px transparent border added around it, the most efficient size is two pixels smaller than a power-of-two size. For example 126x126 will have a border added bringing it up to 128x128, and then that can fit perfectly on any size spritesheet (2 across on 512x512, 4 across on 1024x1024, 8 across on 2048x2048).