There is no such thing as a proper size system. The more volume you have the more stable your pH and everything will be making it easier on yourself is all. The 5 gallons per plant just happened to be size of the 5 gallon buckets, people weren't using cannabis targeted containers. Personally when I did RDWC I ended up going with high volume pumps and using water fall on the return to do away with air-pumps and that was my favorite system in that style. Also eliminated the problem of clogged returns by having the return line at the top of the containers. Wish I could show that system off but those pics are gone a long time ago with the pot site that housed them.