Hi, not sure how difficult this would be to implement, but it would be really wonderful if I could define a rectangle to export to PNG from without having to enter coordinates in the custom-rectangle section (i.e. drag a box across). The custom rectangle option is great, and I use it often, but it's hard to get a good visual representation of how much white-space or how much will be cut after typing in the coordinates, especially seeing as (as far as I know) there's no px grid for the canvas in Spriter.
Great work, though - as it is the program's already fantastic and has reduced the time needed for making animations a lot.