My space is 2x2, x4 high (actually 650mm x 550mm x 1.2m). My recently finished first grow in it netted 4.29 oz. I can see 6 being reasonably done.
I'm using 2 fabric rectangular pots, 24 liters (6.4 gallon) each, that fit nicely in the space. Plenty of medium to grow in with the medium surface being about 5" above the floor. (I don't have the fabric pots on risers, maybe I should?)
The light is a Migro Aray 2 with a recommended hang height as low as 6" above the canopy, so I can make the most of the limited height.