Personally I would go with 4 plants each half if your going to do a proper scrog, it will be easier to train due to the symmetry, each plant having 2x2 to fill. Pick the best 4 of each.
To answer your question, 7gal will do well. Especially if plastic pots and not fabric. I use 5gal fabric in that same space and never had reason to think it wasn't enough, suited my watering habits. But 7gal will be great too. Both will work well.