I think you'll be fine with the 3/4", Sparty. That's why I suggested two of those over one 1" drain. It's only going to be on your fuge so you're only going to have about 25% of your drain water going through it anyway. And again, you're trying to use the best of the space you have since you don't want to have to raise the tank up on a stand. It's going to be tough enough trying to fit lighting over top of it with only a few inches worth of space.
Otherwise I agree with, Acrylic51. Usually bigger is better, but in your case I don't think you'll have to much to worry about.