In UE4 we’ve standardized on decimal units instead of powers of 2, but textures are naturally powers of 2. I didn’t have a particularly strong reason for picking 2.56 versus 1.28 or 5.12, but point of it is to establish a default (can be changed per project and per sprite) scaling factor to make typical sprites a decimal size. In case, a 256x256 sprite takes up 1 meter (100 uu == 100 cm) by default.
Cheers,