The largest I ever saw when when I was in the Peace Corps. This is going to sound like racist fetish porn but its a true story, believe it or not.
I was volunteering on an AIDS bike-a-thon at one point and at one of the stops that was on a river one of the other people who was from Niger suggested we go for a dip in the river. He was about 10-20 years older than me, and as is the habit of the area for someone with an education from that country, he had a bit of a belly on him.
Anyway, I was a little worried about being caught naked by the other local people on the bike-a-thon, so I kept my football (soccer) shorts, which are what people wear for underwear there, on to go swimming. As I turned around I heard this deep chuckle. I swear to god, he said something like, "I guess white people are scared to be naked when they see what a real naked man looks like." Hanging below the man's ample belly was a fat, soft, cut penis much longer and thicker than my five to six inch hardon, with huge low hanging balls to match. It must have been 8 or 9 inches and as thick as a big man's wrist. Honestly, I don't think I've seen anything that big since. I swear I just stood there gaping for what must have been like 20 seconds and I popped a boner which must have been pretty visible through my shorts. He just kept laughing and laughing. He shook it a couple of times and flopped it around and then just dove into the river. Pretty soon more people joined us in the river and I was careful to stay underwater until I got myself..under control. Niger is a pretty homophobic place and I would have been in danger if i had been called out. He never brought it up again, but he always kept smirking every time we made eye contact for the rest of the trip, which lasted about a week.
Before that time, I was never particularly interested in the size of other people's junk, but there was something some powerful, so confident and sexual and, well, cocky about that guy, I still fantasize about seeing his penis again and touching it, watching it harden, etc. Ever since, I have been fascinated by big ones. I don't like SMH - or being a jerk to people in general, but still, there is something so sexy about the obvious size difference - I just can't explain it.