That's a totally subjective question. Size ratings, as given by the manufacturers out there are really based on nothing. It has to do with your load, how often you do water changes, how often you want to refill the reactor, and many other factors. Basically, I suggest going with as large as you can, so you have to refill it less often. Also, the more media in it, the more stable it seems to be over time.
Let's also remember that the point of this project is to learn how to fabricate with acrylic. The reactor is just the case study. If you don't like the height of the one you make, sell it to the LFS or on Ebay, and make a new one ;-)
