I'm going to have to disagree, this is not something that "happens all the time". I suppose if you're a swinger it can be common place but the women I know wouldn't be all too keen on some curious daff bitch asking about her guy's junk. If some woman ask me how big my boyfriend was I'd tell her, "why the fuck do you want to know?" or to the point, "fuck off." The women I know certainly don't sit around chatting about their man's penis size. They may hint that their guy is great in bed during a drunken girl's night out to one up their friends but that's as far as it goes. Asking for the intimate detail about someone's partner is fucking crass and rude. A ho like that better back her skank ass the fuck up as I'm far too jealous/possessive a girl to let that pretentious pig flag be waved in my face.
Think about it, how many of you guys sit around with your buddies and talk about your significant other on a sexual level? I highly doubt most men tell other men what their women have to offer sexually. I would never tell another woman my guy is hung. Why the fuck would I do something as stupid as that? I've learned from personal experience that there is an army of women out there with the conscience of a hungry wolf and the morals of crack strung out prostitute eager to jump into bed with any number of men who are already spoken for. Women who will pretend to be your best friend and then go behind your back and fuck your guy in your bed. Why would I then provide them with information that may encourage their reprehensible behavior?
To the OP, I'd really like to know how your girlfriend reacted?