This has happened to me three times in my life, and I'm a really frequent bottom. I've thought it happened when it hasn't, and each time it happened I didn't know what to think. The first time it happened I was at a bookstore and this guy was fucking me in the booth. His dick was every bit of nine inches, and average to fairly thick. The position i was in was standing, bent over. He was my height, and was in an angle downward. I'm certain he was cumming in me at the time, bareback. As a matter of fact, every single time this happened it was bareback. At first, I thought I was going to piss. Then the sensation of needing to piss in my abdomen became extremely intense. So intense that I started telling him he had to stop, but with him being so close to orgasm himself (or perhaps knowing more about what was happening than myself) he didn't stop. The next thing I knew I started saying, "Oh shit! I'm pissing!" Except then I started to throb instead of relaxing and I got a little frightened. That's when he got off, and I noticed I started pumping out come. Huge load! And, then he asked, "Did you piss?" But, I answered back, "No." Which caused him to look and he was like, "Oh." It was embarrassing, but totally worth it. The next time was pretty much the same, but the last guy to have been able to do it was in the stomach down, legs together position. He had an eight inch cock, and he was also thrusting downward. This time though, I cum super hard, and really enjoyed it. I almost did it with my ex, and the position was the same as the first, but there was very little jizz. I think the easiest way to do it is in doggy if the guys cock is straight or curved down. But, if the guy is greater than nine and curves up, then I'd imagine missionary would work best. I think angle and depth have a lot to do with it.
For those guys who aren't seven and plus, don't worry. There's another orgasm that bottoms have that doesn't require that much depth, but girth helps for this one. It's when a guy is fucking you really good, and you clench down with your sphincter muscles and feel like you're about to nut, but only a small amount of clear precum comes out, or you squirt a clear liquid that's nowhere near as thin as piss. They happen repeatedly during a sexual encounter. Also, I've done this every time I've had sex, with and without condoms.
I hope this helps, and doesn't turn all of you off to me... I love me some large penises, and yes that's plural for a reason.