HI everybody!!
I think the best way to do, is to create virtual resolutions in PowerStrip
For example, for CPS1, if the native resolution of screen is 1280x1024, in powerstrip create a 1152x896 resolution, inside 1280x1024, then in Fburn, just select this virtual resolution, and stretch the image to max
The only problem, is that I don't found any way to save the resolution for every system (neogeo and CPS1 for example)