Thats a really good question. Unfortunately, I dont have an answer for it. Is there something specific that you are attempting to remove (certain cations, or anions, or solids?).
A larger unit size is typically more efficient than smaller units, and the efficiencies depend on many factors 1) temperature, 2) type of elements to be removed, 3) flow rate of the equipment and probably other factors.
Could you help me out by providing a little more information?
Thanks for your help.