According to this study, 8 inches is optimal for woman to cum:
The phallic findings may be reassuring to some.
nypost.com
Penis size for women is mostly about aesthetics.
When we put that against the orgasm survey, women say that they cum easiest and most frequently on their own.
Usually women focus on their clitorises for orgasmic stimulation and they usually use their fingertips to massage their clitorises.
Women may use dildos in masturbation but dildos are not necessary for them to cum.
Penetrative sex toys come in various sizes. If 8 inches was the requirement to cum, then 8 inch dildos would easily outsell every other sex toy on the market.
Women do indeed like penetration and they want to feel that penetration so the size of the penetrative object does matter.
For the purpose or female orgasm, the primary stimulation necessary is mental.
Women like how big dikks look, just like men like big titties and fat asses.
Titties and ass don't make us cum. The aesthetic arouses us and makes us more sexually enthusiastic about that person.
dikk penetration alone is usually not gonna bring a woman to climax.
Basically this survey asked women "what size dikk would make you cum?" with no scientific data to prove it.