Valkyrie: Interesting topic... I agree with wvalady - if/when things start to get hot and heavy I kind of like an advance warning (if you have the goods to back it up of course...). In my case though, it's more likely to cause a positive reaction...
It doesn't have to come off as bragging or assuming something is about to happen. If you're in the stage where you start suspecting something might happen (kissing, fondling, conversation turning slightly raunchy/sexy, visible interest from the woman, heavy flirting etc.) it doesn't take much to convey the message. It can be as simple as leaning forward so only she can hear you and having a conversation along these lines:
"I just have to ask you a very straightforward questions, is that OK?"
"OK." (which is the most likely response if she's interested in you)
"Do you like your guys bigger than average?"
Depending on her reaction and answer you take it from there. From my side the natural response would be a question back as to how much bigger than average - and voila, there you go. Other responses might be an awkward silence - she's either scared, stunned or didn't imagine sex at all - you'll probably have to move on if sex that night was what you had on your mind. She might be a bit embarrased but still interested - you might have to work on it, but at least the message is conveyed.
Important from your part is to _not_ be ashamed of asking the question - no blushing, no excusing, no stuttering, no assuming that she will either accept or reject your size. Just as stated in the first line - a straightforward, curious question.
Also keep in mind - this is based from my own cultural context - Scandinavian. Having spent quite a lot of time in the US though I would assume it works quite well over there too.
/Val