I would agree that most HT's are below 20x20 in size. I don't believe most homes are/were built with extra rooms that size. Unless you were to find a house with an unfinished basement and design something yourself. Or build a home from scratch. You could also add-on to a house. I have seen pictures of people adding on structures to their house specifically for HT. I wouldn't say it's the most attractive looking structure from the outside..hahaha doesn't flow very well.
Good speakers, well placed acoustics, a solid projector with matching screen and a good processor will go a long way. You can save a lot of money by either building the theater yourself, making all the theater accessories and not spending an arm and leg for movie poster frames, special lighting, seats, cables, etc...