Bebo, couldn't have put it better myself.
JWG, just get her a 12 bore, get it fitted properly and a decent recoil pad put on it and use 21g cartridges. If you get her a 20 bore, you will only end up buying a 12 bore in a years time, don't have the hassle of it all!
Im only small, 4'11" and don't weigh too much and my hubby was advised to get me a 20, only to change it to a 12 within the year. As the others have said, you soon get used to the gun and the last thing your wife needs is to get used to one gun then change it later, the sight picture, weight etc will be different and she will have to start all over again. If she really can't handle a 12 then go for a semi auto.
I really don't know why shop owners try to get women into a 20 bore, maybe their wives don't shoot but there are plenty of women on this forum and I would say the majority of them all use a 12 bore. The most essential thing is the fitting, get that right and all will be fine.
Good luck anyway.