A woman's vagina may be between 3 to 7 inches in depth (and can extend after childbirth), hence, an average sized penis (6 to 7 inches) is plenty to do the job
If your manhood is too long, it will push up too hard against her cervix with powerful thrusts and cause her discomfort (which reduces her pleasure). {John Holmes could only get half an erection and Long Dong Silver would sometimes faint when getting an erection due to blood supply etc.}
In terms of circumference, average again is sufficent. For when do you stop? If you keep wanting to stretch her more and more, you will end up with fisting and not feel as much pressure around your penis when you enter.
Concentrate on "technique".