I have been messing about with RetroPi running on my Raspberry Pi 2 vrB, at runs stuff very good imo. I do have a problem tho, I am running the Pi via Composite into my Commodore 1702 monitor and I have massive borders top/bottom. I have looked in the setting and messed with the resolutions but they are still there.
Not sure if this helps but I have a hdmi monitor that is 1280×800 and switched my resolution from 800×600 to 640×480 and it fills the screen 100% now. I may of had to change one other setting I can’t remember though.