I've had 'hands on' on a very good pair of 40Fs, but they may have been bigger; I'm not an expert and it was a long time ago.
Strictly speaking, she was a 'civvie'; she was actually a stripper in London who I got on with quite well.
I happened to mention that I was going to Paris for a long weekend with some mates, and asked if she wanted anything; she said to get her some lingerie. I told her that I didn't know her size and she quickly replied 40F (she might have said 40FF?). As I knew her so well, and could be trusted (hah!), my private dances with a bit more special than usual.