A lot depends on the gun, if you are woried about recoil. A 12 with some more wieght to it than a small 20 will absorb more of the recoil, resulting in less felt recoil. Maybe a ladies model in 12 gauge. If it is a youth 20, particularlly a single shot, I would think the recoil form a 3" shell would be punishing. Whichever you choose consider getting her one of those strap on shoulder recoil pads she can wear under her camo. That should help cut any potential pain, making the experience more enjoyable and increasing the likelyhood she will stick with it and become a permant hunting partner for you.