As big as you can fit. Zones should be based upon the total sump, divided to hold the skimmer snugly and then the rest is refugium & return. Avoid a tiny return zone, those are nothing but a headache.
Knowing of Marc's fondness for acrylic.. he'll probably suggest that! Unless you know how build with acrylic well, I would recommend on glass for the cost alone as having one made for you won't be that cheap. If money isn't much of a factor I would recommend an acrylic one for for it's extra flexibility, strength, and weight.
Thanks for the help. One more question, is it better to build one from acrylic or use a 20g long glass tank? I have been looking at all kinds on the internet. I want to do a fuge. How big should each compartment be?
For a 55g, you will need a narrow sump usually, something maybe 10" wide front to back. The longer the better. For that tank, an submersible pump like the Eheim brand would be ideal.