Just go with 5 gal per plant and your life will be easier....bigger is even better.

Small DWC setup....lots of tangled roots, better be growing from clone. The more water you have the longer you can go without a res change, you might think its big enough till you see how much they grow in the right setup.....I would just buy a few 30gal rubbermaid's and put 5 in each, also you have to think about how your going to control the temperature...iv stared using coolers as a res, great insulation cooling is easy.