There's that brilliant scene in Big where Tom Hanks wakes up in the body of a man and goes to the mirror, looks into his underwear and says "wow!"
There's a similar scene in "Spider-Man" after Peter Parker has been bitten by the spider. His Aunt hears him banging around in the bedroom (he knocked over the lamp trying out his web spinners) and knocks on his door and asks if he's OK:
Peter: I'm fine, just going through some changes...(looks down into pants) BIG changes!
A couple of TV ones:
The sitcom "Still Standing" did an episode about how large Brian, the teenage son, was hung. He was embarassed in the locker room and the parents thought it was because he was small or a "late bloomer" when actually he's a future LPSG member:
Bill (father): You can't let it get bother you, Brian. Everyone is different. There's always going to be somebody swinging the bigger bat.
Brian: Yeah.....but why does it have to be me?
"Family Guy" also did a similar episode about Chris being well-hung where Peter was very, very jealous.
On "The Golden Girls," Blanche was handing out copies of a homemade calendar, "The Men of Blanche's Boudoir," as christmas gifts. Dorothy flips to one page and lets out a huge, "Wooooah!"
Blanche: Mr. September?
Dorothy: Yes!
Sophia: I'm surprised you were able to walk in October.
In "The Nanny," there was an episode that discussed son Brighton being small.
In the Britcom "Coupling," there are numerous references to Patrick's huge package (..."he's a tripod!").