Well 6 inches is considered average, and that's from a medical stand point, meaning the average amount of men in the world have about 6 inches.
I guess it must be true, because for me, 7 inches and bigger is big. I have a small hee hoo though, so that could be why. My ex had 8 and to me, it was huuuuge, sometimes I couldn't even take it all.
You also have to take girth into account, bc sometimes a guy may be a bit shorter (5 or 6) but he is much wider (2-3 incs radius) and that can feel the same inside as a long one.
As far as, "how to use it", well, every girl is different. Some like it long and deep, others not so much. It's all about the placement of the revered gspot, and that can be different from girl to girl. You should just ask the girl you are with, provider or otherwise, how she best likes it.
Hope this helps!