FWIW, I recently bought a 870 and some slugs, and eagerly took it out to the range. The kick shocked the hell out of me! After 5 rounds I was done. So... what to do?

I bought an after-market recoil pad, and man'd up. Went back out, and put about 30 rounds through. The pad really helped. Enough so I finally got the sights lined up properly for me.

Then I played with some #4, some 00 and a "defense" round sold by SG- 1 .650 pellet with 6 00 rounds. For all of these, the recoil was minimal. The #4 was almost a joke, a loud big brother to the .22. The other 2 were still less than either rifle I own, and much less than the slugs.

So, I'd say you'll be OK with a 12-g, even if you're a smaller guy. Hell, I'm 5'8" and about 165... not exactly NFL material!