Reverse osmosis storage tanks never hold as much as the stated size. Reverse osmosis tanks contain a thick butyl bladder as well as a pocket of air. The actual holding capacity of the tank — the amount of water that you actually have to use when you empty a full tank — depends on many variables. These include:
The pressure of your feed water going into the unit. (RO Systems usually stop filling when the tank pressure equals about ⅔ of the incoming line pressure).
The pressure of the air charge inside the tank. (The tank holds less if you put too much air in the storage tank or when your household water pressure goes down while your lawn sprinkler is on).
So there really isn't an exact answer to the question. But a general rule of thumb is to assume about half the manufacturer's stated size. If you need two gallons, get a four gallon tank.
Another point to consider is that you usually use less than the amount stored at one time. And that as soon as you use some water from the tank the system starts making up what you used.
Also, if you need more than the standard tank holds, it's usually easier and more economical to add a second small tank rather than replace the original tank with a larger one. Hooking two tanks together is easy.