How big does my reservoir need to be?

I think I'm ready to try a hydro system for the first time. So I'm looking to build an ebb & flow system. 2 actually. 1 will have a 3'x3' tray. The other will have a 6' x 3' tray. I plan on using net pots w/ hydroton & I plan on putting everything together myself whereever possible to save some money. What I want to know is How big of a reservoir do I need for each tray? And Aside from my Pots w/ hydroton, should I fill the empty space in the tray up w/ hydroton or something else to displace water so I need less reservoir??

I think it depends on a few things
-how many plants
-Run to waste or recirculation systems
There must be some sort of rule of thumb as to how much nutrient solution each plant should have.

I like to put in as big of a reservoir that I can, I do DWC not Ebb and flow, but the bigger the res the more stable your PH and PPM. Also with a bigger res you dont have to change your res as often. My rule of thumb is, big as possible without being absurd.

from the Al. B Fuct threads, which you can double check, but he states he uses about 5L per plant.