Knite,
Currently, the game allows BMP files for gunsights to be 256x256px. With the default setting of 128mil, this means a 256x256 gunsight has a resolution of 2px/mil.
I'm asking for clarification on whether BMP files larger than 256px will be allowed or whether that's going to remain the max size limit. hitech's post was ONLY referring to the size of the sights (in mils) in the game, not the size of the actual BMP files.