Right I see, you were asking about the pots on the next grow then you asked about transplanting to the bigger pots on your current grow.
Id think 2-3 5gallon pots would be more than enough in a 2x3 space on your next grow. Space could still be a problem with that many though. I had 4 2gallons in a 3x3 , moved 1 out so there was only 3 and space is still tight. No room for proper LST. As for the current grow, I'm not sure how you've got 7 in there in 2gallons unless they're really small?
You don't really want to transplant during flowering as it can effect yield if its stressed or problems arise. You should transplant before you flower so you can rectify any problems while its still in veg.