Hey Magus, according to an article on SCROG in the
GROWFAQ, you're supposed to use one plant per square foot of the screen. An experienced friend recommended that I use pretty good sized pots for my attempt. I'm not sure exactly how big a good sized pot is, but I would imagine at least 2gallons. I do know that if you're going to do a soil grow you're most likely going to have difficulty when you need to water and trim the undergrowth so keep that in mind when you set up the screen above the plants. Goodluck.
P.S. 50WPSF(Watts per square foot) is the minimum to grow decent buds, if you've got 100WPSF they'll be much tighter.