Some tidbits I learned over the years...
There are 2 types of female orgasms, clitoral and vaginal. Some women can have one, some the other, and some lucky women can have both.
That said, whatever type of orgasms they get, about 25 % of women have difficulties reaching orgasm, usually caused by their own mental blocks.
For those women who get off on a clitoral orgasm, well, a penis is not even needed, although you do need some skills.
And finally, for those women who cum from a vaginal orgasm, this is usually caused by the stimulation of the so called G-Spot. That particular spot, on average, is about 2.5 to 3 inches from the vagina entrance on the front wall of the vagina cavity. If you insert your middle finger in a vagina and slowly touch the front wall going from left to right while going deeper you'll eventually find a more, how should I say, spongy spot. There you wanna go man. The spot may be smaller or larger, depending on the woman. But it is there, that's for sure.
So basically, anything at least 2.5 to 3 inches can make a vaginal orgasm type women cum, including a penis, of course.
Hell, my index finger is a bit shy of 3 inches and I can make a woman cum with it...
Yet, again, some skills are required.
You must have heard of squirting women. If you properly hit the G-Spot, some women can indeed have an ejaculation of fluids. I was lucky enough to have 2 of those women. What an incredible experience that is. One of them was having so hard an orgasm that the first time it happened, I almost called the paramedics because I thought she was having a seizure or something.
So, as Bindi says: guys need to stop worrying so much about their size and focus more on technique (penis, mouth, and fingers) if they want to make a girl cum.
To that, I would add a very important aspect. Sex is sex, agreed. Yet, sex between loving people has a different dimension. If both partner actually have that wonderful connection, heaven could be real close.
A side note may be, possibly valuable for less endowed men like most of us here, if your erection has a curve, you might want to pay attention to where the head is pointing in order for it to reach the famous spot. If you're curved down there is no point fucking her from the front and inversely if you're curved up, it might be more difficult to fuck her from behind. If you're curve is sideways, you better get some gymnastics skills as well.