[NOT A TA]

Many factory seatbelts are held by a bolt with a shoulder that bottoms out allowing the seatbelt to rotate on the shoulder while others bolt a retractor solidly so it will not move and do not have the shoulder. So some require an original or reproduction bolt, while for others you might be able to use readily available bolts.

As for size, if you gave a clue as to what kind of car you're talking about you might get an answer.