Only a dealbreaker if he's lied about it beforehand. I'm a dyed-in-the-wool size queen, but meeting someone in a bar, or any other situation where there was no overt discussion of size, it's a crapshoot and your own personal preference is no reason to be insensitive to the guy who also took a gamble with you. See it through, have fun, and you might be surprised.
If you're that hung up on the bigness, it's your own responsibility to ask beforehand and save everyone some embarrassment, confusion, humiliation or cab fare. What, you're uncomfortable doing that? Then grow up and stop being so shallow.
Now bring on dat big ol' man-syz diq and all u micropeens fall back.
See? Easy!