There were a few glib answers, and some couldn't quite believe the question, while others expressed distaste for the topic, but no haters.
Seriously, though, maybe you can do a little reading up on a woman's anatomy. Big dicks don't really stretch a vagina out. That's a myth and I always wonder how anyone can still believe that.
As Magdlyn said in another thread:
Originally Posted by Magdlyn
Womens' vaginas are made to expand and contract to accommodate anything from a finger/tampon to a 10 pound baby. I've had plenty of sex with multiple partners in the same week, from a thick 8 or 9 incher down to a medium thickness 4 1/2 incher... with no difference in my vagina being able to expand or contract to envelope either size penis comfortably and erotically.
Vaginas don't "stretch." If they did, my 10'2" firstborn would've made it impossible for me to keep a tampon in post-pregnancy! I find frequent sex/masturbation keeps my vaginal muscles so toned, I don't need to do Kegels. I can get tight enough when I cum to pull a condom right off! I need to be careful sometimes, heh.
If a smaller dick doesn't satisfy me, it has nothing to do with me having been fucked by a bigger one a day or two beforehand. It has nothing to do with anything having been "done to me," but just about how I am built. I need a big one with some girth, end of story. Otherwise I can't feel it, because I am pretty deep and can accommodate a pretty large dick. So, if you are satisfying your wife now, I wouldn't worry about it if she has a lover who is bigger.
The questions remains, however, how your wife would go about choosing lovers according to size? Logistically, she would have to try one first, before rejecting or accepting. But it does reduce the choice of having lovers down to just sex, and that is not the focus nor point of polyamory. Or are you just looking for sexual partners with no emotional involvement? If so, polyamory is probably not for you.