firsttimers:
I'm about 5.5 inches erect
5.5 inches is average, but flaccid you look tiny. Of course, if you are the only man your wife has been with then she doesn't have much experience of penis sizes, except photos and porn maybe.
A woman I know is something of a size queen. Whilst my 7 inches is normally classed as above average in surveys I've seen and most women I have been with have told me - without any prompting from me - that I'm big, this one woman loves criticising me for having a small dick. She told me she has had "much bigger", including the last guy she was seeing, who was "huge".
Good luck in your search for a lover for your wife, but be careful what you wish for. Once she's been "unfaithful", it's done - no changing her back. Take your time discussing it in depth, maybe use the scenario for role-play occasionally in bed with a dildo as the other man. Not every time though. Don't push her, but let her set the pace. It took over 2 years before my wife did the deed, then another 9 years before she did it again with 3 more guys!
She'll want re-assuring that you are not using this just as an excuse to have an affair yourself or as a way to divorce her and maybe she'll wonder why you'd be willing to let her have sex with another man - "do you still love me?" is a question they often ask.
You need a particularly strong, trusting relationship for this lifestyle to work, but work it can. I loved my wife Debbie coming home and telling me about the young cock she'd just had, but I couldn't ever imagine enjoying that with my first wife nor my current girlfriend.
Peter C