Honestly, it's all in personal preference. You have some women who are "size queens", and some who aren't.
From my experience...you could have 4" or 8", if you don't know what you're doing with it, it's useless to me.
I've had smaller guys that had me gasping for breath when I hit my climax, and guys on the bigger side that had me reaching for my silicone friend to finish the job. Vice versa.
Do your research, find out what she likes. If you're in a position where you feel like you're lacking in one area, get creative and make up for it in other areas.