If a gentleman is bragging that he has a particularly large wang, would it be appropriate to suggest that (since a longer penis allows for insertion of sperm deeper than the “smaller” competitors) it is safe to surmise that he comes from a long line of rapists and whores?
Curiously yours
“Why don’t you whip the little fella out?”
“Oh yeah? Well my dick’s so big that it claims me as a dependent on its income tax return.”
Personally, it may be the time of day, but I don’t see a connection between rapists, whores and big dicks. I also don’t think rape is particularly good fodder for comebacks or humor.
I think the OP’s suggested retort may be the worst “comeback” I’ve ever heard. “Takes one to know one” would be better than that.
When I hear a man bragging about penis size, I generally say “My husband’s is larger.” That usually shuts them up.
I usually say, “Funny. Those are the exact words your sister used last night.”
It’s even better if you say it to your brother-in-law.
friedo… thats genius.
or ‘you lie. that’s a big car…’
There was just one topic on the board index between this one and Oh, My Freaking God! (Looong!).
Reminds me of what I say when someone says they are a vegetarian:
"So, no oral sex?
Cause you see if they are so big it won’t fit in a mouth.
Guy 1:“My dick is really really big.”
Guy 2:“Your nose is getting bigger … look, folks, it’s Pinocchio!”
Or something like that…
As one who frequently makes use of this one “Damn my dick itches proceeds to strach ankle” I can say there is no proper comback that can be pre planned. You really need to just shoopt back with something like “Yeah, I kow how that feels. But mine gets numb casue I have ro wrap it a round my leg a few timess” Or simple “your mother” the comeback of all comebaks
How about: “It’s that big? No wonder it gets all the brains!” 
Howzabout: So why do you drive the really big car then?"
“your mother”
See . Perfect comeback 
My usual reply is: “Spit it out, it isn’t yours.”
If you’re a guy, and you’re SURE he’s straight, act gay!
Ask him for more details, tell him how much you enjoy big penises for (insert gay sex act here). Describe what you will do to him in return… that ought to shut him up!
My reaction? “Yeah, whatever.” Followed by me seeking conversation elsewhere.
“Well at least you have one thing you can brag about.”
“Gee, with a package like that I’d think you would do better with the ladies.”
“Hmmm. I guess their giving them out to just about anyone these days.”
“I guess with your I.Q., its nice to have something to be proud of.”
“Say, I should introduce you to a friend of mine…his name is Bob”
“Ahhhh, this explains why you drive that piece of crap.”
Having received several emails with a picture with said ‘large member’ attached, my response is usually either:
“Gee - that looks just like a penis, only much smaller”
or
“The last time I saw something that size, it had an eraser on the end”