They vary a lot...Most of the size N rings I've had from Rocks, and from Gems tv, have been a bit too loose on the 3rd finger, but won't go over the middle finger knuckle. A few of them will, so I have to then decide which ones I can wear with them on the 3rd finger...It's mostly the huge rings that fit the middle one...Some of the smaller settings for some reason also are small in size - the 8mm garnet solitaire is a nice snug fit on the 3d finger, as is the little blue topaz "buckle" ring, and the multigem one with the oblong stones. But that means I have to find ones that go with those, for the middle finger...I've also got the big loopy silver one with spinel - that's huge and even fits my first finger...
So I can only ever wear at most two rings at a time...unless they're stackable ones...my left-hand fingers are bigger, and only that loopy spinel one fits any of those...