For me, it's my pec line. She's gotta be at least up to the bottom of my pecs...any shorter, and I'd have to bend down way too much to kiss her...and that's just a pain. On the other end of the spectrum...I don't see any reason for a limit for tall girls. In fact, I'd love to be with a super-tall woman....imagine having sex and your face is naturally buried right in her boobies? That'd be a helluva trip.