I preferred the smaller of the two I have experienced! The 8" one was a little too much in certain positions, so we couldn't go as crazy, wild as we might have. I'm glad he wasn't too girthy for me because that would have ruined it.
I know the length because I know the size of my hands! When you are playing with them & giving a blow job, you can see the size relative to your hand, there's no need to get out a tape measure.