Technically, he only had to have sex 60+ times with one woman and possibly a few of his daughters to achieve this. Being the father of all men and women, his sperm would be pretty ripe and would likely impregnate a bitch from fifty yards. Also, his daughters aren't likely to complain that daddy's cock isn't big enough and Eve hardly had a wide selection to choose from.
I put it to you that he was average-sized in the trouser department. If all cocks come from Adam's cock, then his must be the most common denominator where deviations either side would result in the larger and smaller varieties of cock that we have today.
Not true. You forgot that Adam was specifically formed by God in a perfect world before The Fall.
If he lived substantially longer his penis might have been substantially longer as well.
Hopefully I get less joke topics than "How big was Adam's cock?"