So surprisingly several different people have asked me this over the last couple weeks: yes size does matter. which is why your cock is almost certainly big enough to make a girl cum. 

See, if you watch a lot of porn -and a lot of you guys do - you end up thinking anything smaller than 7-8" is small because most of the pro guys in porn have big dicks. But just like your average girl not walking around the block in 7" stripper heels, the average guy doesnt measure that much. And that's really great because an 8" dick can be painful when it's all the way inside of you - I know because I tried. (also, walking around in those tall heels is hell as well)

Most my partners measured around 5" (not that I walk around with a ruler and ask to measure their cocks, but in a few cases we did it just for fun) and that was really perfectly good. I'm sure at least one of my BFs measured closer to 4", and he had no problem making me cum (he had a problem of being a jerk, but that's another thing entirely!) 

So seriously, please relax, you're perfectly fine. Sure we joke about it behind you a bit, but that's not different from you joking about our breasts being to small/too big/ not perky etc - and I'm fairly sure that's the only thing guys talk about when girls are not around :)