One point to note that in addition to room size, the placement of the speakers in the room is another big factor that impacts soundstage. Most good speakers are capable of big soundstage I have found IF matched to the room and set up well.
What varies more from speaker to speaker I find is how well or how realistically the speaker places things within that soundstage. BEtter speakers are able to do this better in general. Also, a larger soundstage makes it easier for the listener's ears to triangulate and determine differences in location of the musical elements that occur within the soundstage and together form the music and also to detect subtle nuances captured in the recording.
A large well defined soundstage with good imaging is like watching an HD movie on large format TV and provides the best playing field availble in the home for having a shot at reproducing and our ears detecting everything the recording has to offer.
