Well, I don't know about the size limitations, but regarding format I have found GIF to be the best and clear for schematics because normally we all post schematics in monochrome and thats where GIF compression work out at its best giving out minimum size of file.
yes, i agree about the .gif, it is best for internet.(not such quality on colour though)
and about the size it depends on what circuit you got, usually programs export the picture acording to the size of the schematic, so it depends.