i wouldnt go under a 3gal smart pot for an 'average' plant. edit: by plant i mean marijuana, some of us like to grow other plants too
Generally I figure roughly 1-1.5gal/ foot of plant height. So for you probably 3-5 gallons, just buy 5 gallon pots and fill with soil to the level you want.
i use 4'x4'x12" beds made out of smartpot material. Not sure how many gallons but with 16 plants in a bed i would say its comparable to a 3-4 gal per. the plants love it The roots are so free and rampant. the OG roots have penetrated through two layers of smartpot into the neighboring bed. with maximum root space, i focus on overall canopy.
It would be helpful if we knew what strain you're growing. I have a 5.5ft high tent and it gets tight in there even with 2 gallon pots, and growing auto's vertically. GL