It depends on what method of circular knitting you want to do. You can use one 16" and double pointed needles for the top, or you can use one 32" (or longer) circular needle and do magic loop, OR you can use two 24" circular needles of the same needle size to do it.
I have 16" needles and I don't use them often for hats anymore. I prefer magic loop now because I can work the hat from start to finish without having to switch to DPN.
There are videos on the all techniques in the video section under advanced techniques.