Size really depends on whether your going to go with a bigger tank or not? I would get something like a 60GPD RO/DI. I have a Kent 60 & 35gpd RO/DI Hi-S units.. I picked up the smaller one for $60.00 didn't need 2 but for that price I couldn't stop myself! It's kind of a backup.. I think melev's site has an inexpensive one that is of good quality, check it out. Good luck.