Hi mate, welcome to the big-boys toy corner

It'll depend a bit on your weight and balance and what you want to do with it. I have the 33" Atlas at 189cm, 123kg and balance-challenged, Great fun and versatile board if you're after a big-boy all-rounder. Mine will carry me and a 30kg kid no worries. A 6'10" 123kg mate has also been on it and declared it "a keeper". It's more than stable enough for most flat water and chop, has surprisingly good glide and goes fine in first waves up to about 4' or so. Highly recommended. Not sure really if the 36" Atlas Extra would be overkill, unless you're well over 300lbs?
Try a few if you can, figure out what you like. If it's mostly flat water you have in mind, you might want something longer & pointy.
Where are you located?