Like a couple others hinted, I personally believes that she's heard about your size and became curious, but with her being a complete cunt by nature, she's trying to get you to show it without actually asking. As someone else said, your question was how to get her to stfu... well assuming what I just wrote was true, showing it wouldn't get her to shut up, but would only give her what she wanted, and I'm sure if that's the case, there's alot more to her wants than just seeing it. I personally would tell her that the bullshit isn't appreciated, and if she continues on... well, I don't know your situation, but view your options based on those (moving in with your girlfriend, finding another place, ect.)