Bayou City Productions (www.bayoucityinc.com) created this great example of a virtual set in a 15x15 work space. A 220 for the key light and a 110 for the fill along with a pair of fluorescents on the background created the look and feel of a much larger set. One of the problems they overcame was spill from the background but some black pipe and drape solved that.
One of the subtle things that give the illusion of a much larger studio is the turn the talent makes on camera. Of course this requires 2 takes and a shift in the background but the effect is excellent.