My sister has two young sons. She says the older one's penis is smaller then the younger boy's, which she says is quite "girthy." The younger boy is much smaller in stature and build than his older brother was at his age, and was born with one testicle as well.

Presumably the younger boy takes after his father, my brother-in-law, who is a pretty small, slight guy himself. My ex-girlfriend saw my brother-in-law naked once (we all lived together at one time) and said he was pretty average, so maybe his son doesn't take after him. My sister's father (my step-dad) is himself hung average. I saw him naked a lot growing up, but he stopped that when my brother and I grew taller than him.

As for my side of the biological family, I haven't seen anyone naked. My biological father left when I was quite young and only recently came back into my life. I didn't think it tasteful to ask him how he's hung. He and my half-brother do have larger feet than me though, so maybe they're bigger than me as well! My full brother is pretty identical to me in height and foot size, so I'm assuming we're identical in size (haven't seen him naked since we took baths together as a kid).