I've actually had a girlfriend or two who asked me this question. I believe I said "take a guess" and left it at that. Or I once answered "I figured out how to make my dick 12 inches long. Fold it in half."

I did once have a girlfriend who in referring to previous boyfriends' dick size would say "he was a size 6." I got the idea that she was taking more than the length into consideration. Her size guess had a circumferential component as well.

I think the only solution is to appraise guys' units with something along the lines of a bra size. Like if a guy is average length but kind of thick, he'd be a 6C. Or if he was long and thin, he might be a 7B. I won't continue as I'm sure you catch my drift.

Here's an idea: Give your boo a nickname which adds an inch to his saber. "Yo, size 8. Get over here and bust me out big boy. I'm in the mood to handle it all." Your sex life will only improve with that approach.