you can grow in smaller containers in hempy pots, but you have to water more often...in a 3 gallon one watering a day is enough for most plants, in a 2 gallon pot, i would water once in the morning and again in the afternoon...
but autos are weird, some can take the same strength food you would feed a photoperiod plant, and some will scorch and burn at half the strength. i would guess that in two gallon pots, you would have to feed twice a day at half strength in most cases.
as to the yield? 1.5 x 2.5 feet isn't a lot...i would go with two and feed them as heavy as they would take, 4 plants would never have room in that area, unless they were tiny, which isn't a plus.