The purpose of framing is to help the image speak for itself, so the viewer enjoys looking at the print – not the frame.
Here are some pointers:
Only use narrow frames in small spaces – elsewhere the image gets lost on the wall.
Use 2 cm width at least for a small print; always buy wood, never plastic.
On larger prints frames can be 3 cm wide or a little more – but don’t let it dominate: keep it simple!