If you can make her cum with one FINGER (which you can) then a penis of any size above 2 inches erect will be able to do the trick - you're over DOUBLE that!
Guys have this fixation about how big they are, when actually 90% of girls don't really mind that much. What they care more about is that you do foreplay right, get them in the mood, go down on them, and can stay hard during penetration for about 10 minutes. Once you can go 10 minutes or longer, you're likely to be able to make her cum vaginally (especially if you use positions that hit the g-spot, which is only about 2 inches into the vagina along the top wall), and that is the deepest and most powerful orgasm she can have.
Check out my sig below for David Shade's material on sex - particularly "Give Women Wild Screaming Orgasms." Hell, the guy can get girls to cum just by TALKING to them, without even touching them! Really good stuff. You'll be fine
