16 in are the only ones you can use for hats, the others are too large. You'll also need some double pointed needles (DPN) for the top part when you decrease. If you haven't used them don't be afraid.. learning to use them with hat is the best and easiest way.
Just for the record, there are a couple other methods for making things in the round and can be used for hats.. Magic Loop and using two circs. Both methods can be researched on the tabs at the top of the page under advanced techniques. I think in this case the 16 in and DPN are your best bet though.