I am one of those rare ones even among fellow boob-men that think that there is no such thing as "too" big in terms of size.
As long as my partner is comfortable, remains healthy with bigger sizes, I say let them grow!
Don't get me wrong, I would appreciate a G cup too, but i would say anything beyond G or H cup, all the way to the opposite end of alphabet, preferably second half.
UK K or M cup would shut me up. (band sizes 32-36 preferably, would not mind 38)