I have 2 15 gal and 2 3 gal in my 2x2x4 tent. I could also of used 5 gal tall pots instead of the 3 gal, but only wanted 3 gal this time for those ones. If your in soil, the bigger the better.
Yea 7 or 10 gallons for 1 plant. The more soil the better. I run a 45 in my 4x4 (got tired of multi-pot grows and just put it all in 2 big pots perpetual.