the thickness is not related to how fat or skinny you are, but to water temp and dive time.
Do a wise thing: drop a line to Jim at
The Spearfishing Store He's a floridian dealer selling wetsuit to loads of floridian divers, and he's also a great guy!
His website is under developement this days so don't mind the online catalogue and ask him directly: he's got some good special offers on suits.