@martyn22 Recalbox comes standard with the default setting of 4/3. To switch to full screen, go to "game options" > "screen options" and change the resolution to 16/9. This solves the problem.
If it's already set to that aspect ratio, make sure the aspect ratio IN ADVANCED CORE SETTINGS is 16/9. (advanced settings > advanced emulator settings > choose emulator and check).
If you're using recalbox on a smart TV, check your TV's display settings. This varies from TV to TV.