You don't need a huge res for big plants. Check out my LSTTT thread to see what an Emily's Garden setup can do. I had 6 plants in a 2 gallon res, and things were great. The key to not getting root rot and other problems (not to mention huge, healthy plants) is getting air to the roots, the more the better. It is impossible to have too much air in your water. Plants want air just as much, or maybe even more than they want water. Also, change your res as often as you can stand, up to once per day. In the Emily's Garden I feed when the nutes run low, and try to do a complete flush at least once every 3 weeks, but more often is better. Watch your res temps, you may want to consider a chiller. Higher temps mean faster growth but bigger chance of root rot. Lower temps mean slow growth, but little risk of rot. I forgot the ideal res temp, but I just try to keep my environment below 85, preferably 82, and I assume the water is close to that.

And WH speaks the truth about root conditioner. I use SubCulture, recommended by Rhizome.