Last summer I built a "Timesaver" using Super O track, which should be the same geometry of HO track if I remember right. The smallest I was able to shrink it down to was three foot by seven foot. I don't know if the sidings were the proper length, as I never really further than setting the cork roadbed and the track.
I'll try to snap a picture of it this week; I really ought to work on it some more.
J White