A schoolfriend of mine had a pencil penis. Very long, about 8" soft, hanging beyond his balls, but less than 1/2" thick, hence the term "pencil penis" I suppose. He was a shower and it didn't change much when hard (we were quite good friends!)
I think to be a pencil dick it needs to be about average length or longer and thin. I guess I agree with sixofspades sizing...4" circumference and under is skinny, 4" to 6" circumference is normal, and 6" and over is huge. (and being 6.75" around I like that grouping! lol)
I think that if a dick is short and thin it's not necessarily a pencil dick..its just a small dick. there are different terms used to tease about it
I suppose what one considers thin is entirely relative to oneself. I consider any cock under 5 circumference to be a little too thin. Maybe I would consider 4.5 circumference pencil thin. I think of cocks around 6 or 7 girth as average and don't think a cock can be considered extra thick until it reaches 7.5 or 8 girth.
I'd consider it 4 or less too, I played around with a tape measure and it's weird how some size differences seem to make a bigger difference, the difference between 5 and 6 inches doesn't look nearly as big as the difference between 6 and 7 inches, 7 looks MASSIVE.