A few times.
I've been asked by a girl who hadn't had anyone bigger than me, she literally whipped out a measuring tape halfway through and measured me at 21cm. A while later we were working together and on a work trip she whipped out a IKEA measuring tape and wanted my girth!
I've also had 3 girls from the office pin me down at a house party and pull my trousers down. They were doing it as a joke but then noticed the bulge, so they looked at each other and the boldest pulled down my boxers. They were all pretty excited and again, the boldest grabbed my semi and got it hard. One of the others then grabbed a remote control and compared me. I had a reputation in the office after that.
Another girl I slept with measured me in the morning after. Weeks later her friend was in town from the US and she told me I was 'the one she wasn't allowed to talk to'. Anyway turns out my size had been communicated to her and she wanted to see for herself. Ima thick Louisiana accent her reaction to my cock was 'jesus Christ, you come ready to party!'.
In my experience girls always tell. Having a big cock is vital for a girl, they are often more than happy with average and smaller. But a big one generates a primal level of excitement that takes them over. And it's fucking hot to watch a girl grab you and keep grabbing to get a real feel of your size...