It all depends on your available space. I have a 90g and I wanted a completey enclosed system so the largest size I could put in my stand was about a 25g. Now that I am plumbing it the basement, I will have a 25g Refugium and a 40g and 33g sump connected with another refugium in one of the sumps. But I agree that you should go with the largest size you can.
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