Actually, I pride myself in knowledge about underwear...almost to the point of a fetish...but not quite...that'd just be weird.
Anyway, to the OP...
1) Your best choice would be something like briefs or a jockstrap, something that gives very good support and doesn't give too much free motion. A lot of people will suggest boxer-briefs, as they are strongly coming into style and popularity, and offer a good amount of support, but not what briefs will offer you.
2) No underwear will cause your penis to grow more than any other. That is all from Genetics from your parents. Sure, there is some debate of briefs causing a lack of sperm production, or a destruction of sperm, because it doesn't allow enough freedom to allow the testicles to stay at proper temperature, but I don't really think much of it, as long as you don't wear anything EXTREMELY tight.
3) Comfort is completly a personal opinion. I honestly find my comfort in briefs, boxer-breifs, boxers, or freeballing...I also don't mind Jockstraps. Whatever feels best on you, that's what matters.
As for style...well anything could really be stylish...but you don't really care about that...so what does it matter...
There you go HelmsmaN...my expertise...good to be back !