When I met my wife we were teenagers and she claimed to have had sex with a guy that had a 10 incher. She later said I am bigger than him so she has ,as most women, no concept of actual measured size or she is lying
Funny. I've had similar experience where girls had misconception of size, due to other guys lying or not correcting them on their size.
1) A friend's GF at the time (now ex-gf) were fucking, and she later came out of the bedroom, and said "I just measured ___ and he was like 9in. long...I couldn't believe it, I was way off what I guessed. So you must be around the same size... and yours is thicker from what I remember." We had a 3some with her, and were pretty close, which is how she knew my size.
I told her I thought she measured wrong, likely along the bottom of shaft, and she said she did. I told her the right way to measure was along the top, and that I was only 7" long (measured properly), which meant he was that size or smaller (he had a little less length and girth). She couldn't believe it, since he didn't correct her.
To her, my friend wasn't the biggest she had, but she still thought he was big, due to bad measuring.
2) My GF around the time we first met and started sleeping together, told me she had said I was "big/thick" and "well endowed" to her close friends when they asked after first time we slept together. We were pretty open about past sex partners and talked about anything, so I knew she had some guys that were longer than me - as well as some that were average, small, and even one guy she said was tiny (literally the size of her finger and almost as thin).
One day she mentioned that a common female friend was dating a guy that was hung (supposedly 8"+), and we got to talking about size of some friends I was going to hook her up with that were bigger than me.
She asked "How big are you? Have you measured it before?" I replied "Yes. Guess. How big do you think?" She thought and said "I don't know. Not sure. I guess...around 6 in...maybe a little bigger or smaller. I don't know". I took her ruler out, and had her measure it, and she said "7 inches Wow. Really? I didn't know what to guess, but 6 inches is half of a ruler and that seems big."
To her big just felt big, and wasn't a specific measure.
Two examples showing common things that lead to misconception of size.