55 is fine, but bigger is always better. Check craigslist for good deals on 55s and 75s. I bought my 75 gallon sump off craigslist for only $25.
Depending on where and how your tank is set up, the sump doesn't have to go under the stand. My sump is actually next to tank, rather than under it, as my tank was already running when I switched to a sump.