I play with a lot of different sized toys, from 5" (avg) girth to a bunch of 6" and 7" ones, and a few hefty-sized 8, 9, and one 10" girth plug I just bought. Lol, call it a hobby if you will.
I like them all, but in different ways. The large ones you have to go slow and are more about getting a slow stretch than fast pounding. For getting fucked in real life at a good speed, anything up to 6.5" girth is great. Over 7" it starts to get too intense when you really thrust in and out quickly (unless, like me, you warm up beforehand with a 9" dildo.
6" girth is above avg but not uncommon. I'm a little skeptical though about ppl saying they've encountered a 7"+ girth in real life. I have a dildo that's 7.5 and I've never seen a cock (even in porn) that matched it. Maybe those Castro vids or whatever, but thats it. My understanding is the avg girth is between 5-6?
So yeah, I'd say exactly 6" is perfect! and then I can get pounded from all angles while being able to focus on the prostate sensations and stay hard.