OK this might sound silly and dumb, but need help!?

I have alot of female friends and I am in a relationship. There's one pasicfically close to me. We alwasy talk crap to each other but in a cute way I guess. How ever one day we were talking crap like we always do, Im 5'8 and half mex and Italian. so she makes fun of my hieght, calls me midget, shorty and all that. To make the story short, she claimed that I had a small cause im short. Now me being a man I want to prove it to her that I wasnt. So this went on for days until one day we were studying at her house and again started to talk crap, so I showed her, and now she wants . I am affraid cause I see her as almost like fam, I dont want to ruin that, and the fact I have a girl friend (2 months) WHAT DO I DO? Kristy my ( friend)is SMOKIN HOT!