You have two options (minimum) to increase the scale of your items in the ecosystem.
#1 go to the list of items that you use in the eco (first tab in the eco material editor) and change the scale value for each plant individually.
#2 go to the scale/orientation tab in the eco material editor and change the overall scale there. Default is 0.5, maybe you try 1 or highter.
It is difficult to say how much computer memory you would need to do what you wanted. For that there is no formula. But be assured, there is a limit for EVERY system. May it be as big as it wants to be. One part of digital artwork creation is always to find clever ways to make something look good - with as little resources as possible. This is one part that makes it so interesting in my eyes.