Think about how much water you use at a single session of watering or fertilizing (it's probably greater than you think).  If it is more than a gallon or two, the 3-gallon tank that comes with your system likely won't be sufficient, and you'll need to either upgrade to a larger pressure tank or use another method of storing the water, such as plastic drums (available for free at most car washes).

Most folks think that a 24 gallon-per-day system will be sufficient, and want to go with a smaller capacity in order to keep costs down.  We recommend that you not do that, but opt for a system with a much larger capacity - at least 50 gpd.  Our reasoning is as follows:

1. The rated output published by the manufacturer is significantly overstated in practical terms..  Those levels are typically measured at 70°F and 60- or 65 psi water pressure, and most households don't have water that warm or at that pressure.  If you count on 75% of the rated output, it's closer to reality, although water quality affects that, too.  For example, "softened" water, in which the Ca & Mg are replaced with Na via salt pellets, is easier to filter than the original hard water, and you'll extend the life of the RO membrane.
    
2. Think about the "rebound" rate:  if a system gives you a true output of 12 gallons per day, that amounts to a half-gallon per hour, so your three-gallon tank will refill in six hours.  Doubling the output halves the waiting period before you can water more plants.
    
3. The cost differential between a 25- and a 50 gpd system is insignificant.  In fact, our price for the larger-capacity system is are often the same or less, and selecting an even larger system is only a small amount more.

Going with a large capacity system also offers you room for expansion of your collection, although any system can be upgraded by simply purchasing replacement components sized for larger output (they all fit the same holders).  One thing to consider, however, is that if you install a very high capacity system under your sink, the volume of flow while refilling the tank will be significant, so if you're sensitive to water draining noises, you might not like it.