As far as I know, most are more or less selected by being able to get it up and cum on demand, and to go as long as is needed for the scene. Size is secondary to all that.
I'll also parrot what Sean Jacobs said -- Since I have been getting more into PE and paying more attention to things, I'll have to say leaving out the giants, most guys in porn don't actually look to be more than 6.5-7" NBPEL and 5.5-6" girth, maybe smaller. If you want a good example too, google any clips with Johnny Sins, as on Thunders (I think) it has pretty much been confirmed he is in the 6.5-7" NBPEL range and is considered 'big' AFAIK.