I'm wondering...
If bandwidth/server space is the biggest concern, and the software permits it, a size limitation that allows for a greater number of pixels for the longer dimension of a rectangular image would be nice. As an example:
750x750 = 562,500 pixels square, while
850x650 = 552,500 pixels square.
Both examples consume similar amounts of server space, and may use similar amounts of bandwidth (please correct me on this if I'm wrong), but the resulting increase in resolution for a rectangular image is significant.
Matt