An area 24 inches wide will hold 3 spurs separated by large radius switches. Allowing room for the switches, you could probably store 36 forty-foot cars. A Y switch might save some length and allow a couple more cars.
If you intend to use the area as a sorting yard, you will need one of those tracks to assemble trains and a yard lead to back out on. Most people use the mainline for these purposes, which works as long as you are the only train running.
If you are looking for something challenging, the area could be used to build a time saver switching layout.
If you haven't visited Greg's site, you really should. He has a lot of useful information there for our hobby, as well as cigar information. His switch yard is quite nice, given the space he has to work with and he has given me a few ideas to try on my next layout. I am not sure where he stores his trains when he is not running, but given the constraints of space, I think he has done a stellar job.