As was said by others already, never assume that a woman is going to orgasm from vaginal penetration alone. If you do, you are putting pressure on her, and are setting yourself up for dissapointment. And I don't know what percentage of women actually experience female ejaculation ("squirting" or "gushing"), but I've never been with one, or else I didn't have what it took to make it happen.
My experience with female orgasm (for what it is worth): out of a total of around 15-16 partners roughly half had orgasms with vaginal penetration alone (or else faked it well). But of this 50%, only some could do so consistently. Around 3 came nearly every time with vaginal sex, and one lover could only come with vaginal penetration. The rest came pretty reliably from oral sex or manual clitoral stimulation.
Maybe it was my poor technique, but I have never had a lover who really got off on a lot of vigorous fingering in her vagina (although most seemed to like a little as part of foreplay).
As for my wife of 25+ years with whom I have had sex thousands of times, she orgasms 99.9% of times from either manual clitoral stimulation (erect cock or fingers), oral clitoral stimulation, or a combination of both, along with a healthy dose of nipple sucking. She orgasms occasionally with vaginal penetration alone (no manual stimulation), and when this happens it is almost a suprise to us both since we don't seem to be doing anything differently than on many other occasions when she didn't. She says, however, that when we are having vaginal intercourse she frequently has "little orgasms" that may not be apparent to me. We have never used vibrators or other toys.
I agree that it is a real treat to watch and feel a woman having a wild orgasm on your cock, but I don't think you can expect to have that with all gals, and not every time with any. If you want to increase the chances of her coming during vaginal sex I would suggest that you encourage her to stimulate her clitoris, or let you do it. If that doesn't fly, maybe you could try one of those cock rings that has a tiny bullet vibrator to target her clit.
I knew one gal (best vaginal sex ever) who had great pelvic and vaginal muscles and could use them to bring herself and me off with vaginal sex. I have heard some other women say that rhythmically contracting their perivaginal muscles as they approached orgasm was a key to getting them over the edge. So you might suggest she start doing Kegel exercises and try this.
Of the women I have had sex with who did orgasm with vaginal penetration, cowgirl (facing) seemed to be a favorite position with deep penetration, and a lot of rocking and grinding against my pubic bone as they got close. One, however, prefered missionary with her legs up on my shoulders ("jackhammer") and for a couple it was vigorous doggystyle with my balls banging against their clits.
I'm not sure how much penis size matters (if any) in determining a woman's ability to orgasm with vaginal sex, but I am a little over 7 inches functional length and about 5 1/2 inches in girth, for the record.
