Despite all of the talk about bigger being better, at a thick 7 inches, my wife claims mine is the perfect size for her. That said, she still likes playing with other cocks, and two of her previous lovers were noticeably bigger, and she definitely enjoyed the longer deeper strokes and how they fully stretched her.
Her current long term lover has a pretty average sized cock in length (just under 6 inches) and in thickness. She says she actually likes sucking his cock more, because she can fully take him without gagging. His is also the only cock she enjoys in her ass. She's still tight enough that he feels fine in her pussy too, but it's his oral skills keep her cumming back for more!
You don't have to have a small cock for your wife to enjoy cuckolding you, at least that's the way thing are with us.