AFF is full of nut jobs....so check them out, in addition sexual health god only knows where they've been.
Now on relationship side of things, make sure you are both doing this for the right reasons, fun and experimentation, not to fix or put a spark back into a relationship. Then there is the issue of attachment, will she prefer the new guy over you, is his cock bigger, how do you feel about your wife getting a good pounding and having earth shattering orgasms with him and not you, do you have a secret fixation to be cuckold, or put down, made a lesser man!
Buy her a big dildo, make love to her, give her an orgasm, then fuck her with a big dildo, buy a strap on anything but this!
Seriously, I know this is a site and forum for guys and girls that like or have big dicks, but in a relationship, a big cock is the least concern for most women. Ok so I'm going to get slated over that, but I will stand by what I said.
Be very careful, afterall there will always be a guy out there who is bigger, thicker, longer, younger, fitter than you, earns more money than you and drives a better car!
If you want to experiment and introduce someone else, have fun and excitement then go for it, it can be a lot of fun and pleasureable for all concerned. If everyone knows the rules.
From my personal experience my partner made it quiet clear to me and to anyone I am with, do anything behind his back and we'll be feeding the fishes and he'll be spending the insurance!