Um. i would say get the biggest one that you can fit in the area. I would think that a 10 or 20 even is too small. And also depends on what you are going to do with it. Will it be fuge/sump or just straight sump? The bigger the more water volume that you have.
i got a 36 by 12 from www.protein-skimmer.com that fits just right under my 75. mine is a sump / refugium and it come with a skimmer and lights. hopefully this helps you.its on sale for 299.00 or maybe they can make you a sump.
Sorry jumping on for a little input. Figure.out exactly what equipment,skimmer,pump/s, fuge etc.. you are going to use and flow rate then get in touch with jeff(custom king). I think it's the best/easiest way to get the most out of the space you have to use. Just my opinion,he makes great stuff and great prices. Personally I cant see myself ever buying a stock made filter of any sort ever again.