Different positions suit different sized/shaped people. A position that works well for one client is a disaster for another.
Even within one position, variations can suit different people (or combinations of people!). I think I have so far tried about seven or eight different styles of doggy-style.
In my experience, it is not about wang-size or sex-positions - but being aware of how existing physical attributes will work in combination. I find depth often has very little to do with it. To me a good position is about angles, body proportions, a willingness to try, and the gentle humility (and confidence) to smile at each other and be understanding if it doesn't work.