Danger ahead!!
#1) women, genrally, aren't interested so much in the penis as the man it is attached to. (A fantasy about having sex with a big guy is one thing, but generally it is just a fantasy.) Yes a large penis can be an exciting thing for a woman, but once again, it is the the man she is interested in. The big cock can be a plus (so to speak) for some women, just like being well muscled, tall, broad shouldered, etc.
#2) Has she mentioned this to you? If not she might very well regard this as creepy. Some women like to their husbands to be a little jealous (in fact I'd say a fair number of women like it within certain boundaries). So if you effectively tell her that you're ok if she has sex with other guys she may very well feel as if you don't really care about her (even though from how you phrased it, the offer would be because you do care about her). Or, even worse, she'd feel like a real man would be jealous and never accept that, let alone propose it.
#3) Are you sure this is about her and not you? I honestly don't know. But this is a fantasy for some guys out there. I don't get it, but I guess I don't really have to. Some guys get off knowing a "real man" is fucking his wife. If that is you, okay, but understand that it is YOUR fantasy, not hers, so you are proposing this for YOUR benefit, not hers.
#4) I'd say that the only reason you should even consider this is if she has made comments SEVERAL TIMES that she'd like to have a big cock. Otherwise, I'm willing to bet it is either an insecurity of yours or a fantasy of yours.
#5) Lastly, if you do seriously consider doing it, think long and hard (so to speak once again :smile: ) about it. Can you handle the consequences? What if she really, really liked it? (not all that likely IMO, but still a possibility) Could you feel as if you could ever satisfy her again? It is a critically important part of the male psyche that he feels as if he can satisfy his sexual partner. Without that, you don't feel like a man, and a man is what she wants in her bed.