One way to find out....
If a cap is too big, it can be pinched to hold it in place. If it is too small, it won't seat on the nipple, and misfires will result.
If a nipple is too big, it is easy to polish it down. Chuck it up in an electric drill (or lathe if you've got access to one) and spin the nipple against a file or abrasive, depending on how hard it is. Go easy, keep trying a cap until it pushes on and bottoms without too much force, or is loose.