If the issue is length, I think three inches is my minimum. My fingers are three inches long, and after I get into masturbating, I usuall can't reach all my favorite spots after a while. You know. Once the friction is reduced, and some parts swell, and I get deeper. My three inch fingers really cant do the trick anymore and I have to use them on my clit instead. However, I am finding that more more varied types of vaginal excercises I do, (and the more consistently I do them) the more sensitive my entire body is during sex, the stronger my orgasms are, the tighter I am, and the closer I get to orgasm merely from penetration with my fingers. I have always bee occasionally ably toget myself off that way, but not without a lot of difficulty and concentration, and sore wrists after. It's much less difficult now. As for girth, I really do like a full, stretched feeling, so I do have a special place in my heart (and vagina) for a very thick, very firm erection. However, I'm small inside, and can get more than enough pleasure from a circumference of 3.5 inches. I don't think any smaller than that will do for now, because I get extremely wet, and I cant get enough friction with something smaller for long enough to get off at this time. I'm always working to improve my vagina though. I think a lot of women have lackluster experiences with smaller guys and are too lazy to take any responsibility for their own pleasure. My mother taught me that during sex, everyone is responsible for the pleasure of the number of people involved, including themselves. That's not what she said, but a lot of other things she said led me to that conclusion.
Anyway. I talk too damn' much. To summarize, 3"x3.5". This is the current minimum.