Having just watched the first edition of "Big Brother Adults Only" in Australia, and having read some of the online forums, it's pretty clear that Jamie "wins" hands down. He looks he's about 15cm flaccid, growing, according to on online forum to 21cm (just over eight inches) which sounds reasonable.
While David looks to be on the larger size of average, most of the guys (Ashley, Dino, John) seem pretty standard average, while Michael (who looks like he might have some Asian heritage) is on the smaller side of average. No spottings yet of the other housemate, Gaelan, though I reckon he looks like he could be Jamie's older brother, so who knows.