Bigger may be better- but I'm doing my first AG batch on Monday, and I'm doing it in my 30 qt. SS brewpot. I had some doubts about the size, but after going through as brewer's assistant to my son on his AG batch of porter using exactly the same size and brand of pot, I'm confident that I won't have any trouble (this is starting the boil with a solid 6.5 gal.)
I plan to back off on the fire as soon as a boil is reached. I see neither the need nor the desirability for the "volcano" boils shown on some of the You Tube videos. This means that as the hot break occurs, the foam is easier to skim off. Of course, I've got some Fermcap-S as insurance.