I'm sure there is a rule of thumb about this, but I don't usually go by those unless I know nothing about it. I usually just guess at what seems right, kind of like the basic rule for determining what speed to drive. This way usually works out quite nicely for me because I tend to over do instead of under do. (An ounce of prevention and such).

I think the gallon per rule is probably more per plant than per area of screen, but I bet there's rules both ways, I just don't know what they are. In general you want as big of a container as you can stand using. If the plants get root bound under your scrog, once everything is all tied down and grown into the wire, you are screwed. It will be a total pain in the ass to change them. If I had a 3 x 3 area, and were growing 8 plants, I would divide that area by 8 and try to get pots with the tops that size, so I could fit as much dirt under the screen as possible. It would also help to get deep pots.

Sorry I don't know specific numbers. Hope that helps a little.