What post size should I use for a 20'x24' deck
I wouldn't even consider using a 4x4 post on a deck that big unless it was more of a patio.  First most code requires 6x6 or greater for decks.  Then you could have some huge warping issues with 4x4s - if you used them and they are more than a couple of feet you better strap then.  Also you can notch the 6x6s for rails and stuff - you won't be able to nothing a 4x4 without taking out its whole top.  This doesn't even get into strength comparisons.