Besides the size of the shooter and stock fit, there is another factor in managing recoil, and that is shooting "style".
And by that I mean there are other effective ways of shooting the shotgun at close range, other than mounting it on the shoulder.
My mother, at 4' 10 (and a HALF!
) never weighing more than 109lbs in her life, wearing a size 3 1/2 ring, could handle her daddy's 12ga Ithaca double barrel. And quite well at close ranges. She shot it from the HIP!!!
With the gun held firmly in the hands, recoil just makes it "swing" back a bit. While I don't think any current defensive school of thought promotes this method, it can, and does work for many people, and you might find that a smaller, recoil sensitive shooter will practice more, and learn to be good, more easily with the gun at the hip than on the shoulder.
The fact that they aren't being battered by the recoil could mean they shoot more, and become more skilled. Do NOT dismiss someone who is a skilled hip shot. They are neither unarmed, nor ineffectively armed. At home defense ranges, they can be highly effective.
It does take quite a bit of practice to become a master at hip shooting, true. Some people are "naturals", and others can learn. Without the punishment of recoil, the learning can be easier.
12ga. No question for me. I don't even own a 20.