Welcome to the Swingers Board
For one thing,I applaud your wife for trying, and speaking up to say enough is enough for another. It's a very important thing in MFM play. Supporting her decision, on your part, is well deserving of appreciation also.
Without knowing your future choices I hope you at least know whats possible or not. If its too big, its just to big. There is nothing wrong with that, we aren't all a perfect match.
Had you asked the gentleman about his size before meeting ?
Or, were you both in agreement about something different for this first try ?