I think the sizes mentioned above, while not impossible, are extremely rare. Most people are terrible at estimating distance, measurements, etc, and I think that applies to penis size as well.
Just what is a soft penis anyway? As guys, we know that sometimes we spend a good part of the day with a chubby. A chubby can add quite a bit of size without being fully hard. The only penis I have measured is my own, which is about 4.5" competely flaccid. So that is what I use to compare sizes. Not really accurate, but if a guy is obviously bigger than me, he would be at least 5" +.
I was on my high school wrestling team, and regularly saw my teammates naked during weigh-ins, in the locker room, and showers. Most of the guys seemed to be roughly the same size. A few, mostly the younger ones, were definately smaller, and there were some, like me, who were bigger. But one guy who I wrestled with jr and sr year had by far the larges penis I have ever seen. He was at least an inch or more longer (making him about 6" soft) and thick. When he would walk to the shower room with his big penis swinging back and forth, it really was a sight to see. The rest of us were really quite impressed, and one time someone asked him how big he got when he was hard. He could have told us 10" and we would have believed him, but he said he just got to about 8". He was a great guy, and not at all cocky (sorry about the pun) about his size.