It also needs to be big enough to hold any back siphon from your tank when the power goes off. I agree with as big as possible too. You will just likely need more room than a 20 gallon tank for a system that size.
I've got a 40 gallon breeder but it's got holes at the top cuz the guy iv got it from use that for his overflow would this work if I found some way to plug up these holes
I would figure out what equipment I would want, like what skimmer will be used, return pump, fuge (if I wanted one), reactors (if used) and make sure I will have enough room for what I want to use.
All depends on your budget! I would do a custom acrylic just because I have the ability to make them myself. But yah I would get something in the area of 35 gallons atleast and highly recommend a refugium involved somehow. Mine has been awsome.
Wish I could make sumps but my job that funds my life takes up 60+ hrs a week and I have 3 kids so I never get time for side work. Wish I could. There's a awsome sump for sale on this site right now Idk if it will fit your stand though.