I use a 20 gallon recycling bin. It's filled with 15 gallons of water. Keep in mind that a 10 gal fuge should only have about 6-7 gallons (or something close to that) of water in it. You need to leave some excess capacity in case your plumbing malfunctions. This setup has worked great for me so far (7 months now). It has provided plenty of room for Macro algae growth and live rock rubble.
I really don't think your fuge needs to be the same size as your tank. That being said, a fuge that large would certainly be very effective, but probably not very economical space wise. It'd be tough to fit under a 55 gal. I guess you could plumb it in from a different location.
Make sure to keep researching this topic (as many sources as you can find). I'm guessing you'll be able to get a pretty good feel for what you need if do.