I'm pretty sure that a 20 gallon, long talk would last a corns whole life, or at least that's what I'm told.
And in my opinion corns are a great starter snake. If it helps at all, I'm 17 and I got mine at the end of 2002, and I've never been bitten or had any real problems with her. I think that they're a great beginning snake. I use an under tank heating pad for mine. The thing that I consider very important with corns is to give them multiple hides. Mine has one that she likes better, but I always keep 2 or 3 in there for her.
As I said above, I've not been bitten by mine ever, I think that their temperment is good. I have her and a ball python and a spotted python and I think that she is my nicest one.
Good luck with your decision.
-Drako