I would get them small and watch them grow. You save a couple bucks and it's more fun that way. The only fish that I would buy with some size on it would be a trigger though. They grow realllllly slowly. I would start with one 4-5 inches.
One other thing--I used to have it open top, but for 12 bucks, you can get egg crate from Home Depot and cut a nice top to fit your tank. I have it on my tanks. I have heard too many stories of losing a prized fish after years of keeping it. That has to be devistating. But for a little bit of cash and 20 minutes of your time, you can have a perfect fitting top, still have the openess, and keep a fish or eel from jumping to its doom.