Half a gallon should be fine for now, When you transplant I would strongly reccomend getting a high P solution to help minimize the stress from transplanting. I just transplanted, and mine is having a tough time. But switching from pot to pot is much easier then ground to pot. Hopefully you have read the FAQS on transplanting. Are you going to double pot? or totally transplant? If they have superthrive near you, I would get some for the transplant, helps the plants alot during transplanting. They have it at wallmart, lowes, and homedepot. Good luck man : )