For a BIG SOUND, 700-1000 square feet, with 10 or 12 foot ceilings, if you can swing it. Use the golden ratio for dimensions
Excellent advice. The best sound I have heard so far was in a room of 26 feet wide x 23 ft long x 10 ft high:
Jim's Powerful Sound

The main reasons why it was the best sound (even though not perfect to my taste) were:
1) a large space that allowed for an illusion of the power and dimensions of a full size orchestra, as well as breathing room for a really open sound
2) speakers that could energetically and effortlessly fill the large room. A lot of hyper-expensive but, relative to those, small speakers don't need to apply.
I wish the ceiling had been 12 feet high and the room longer (would have allowed for more depth behind the speakers). If I had the funds I'd probably start with the idea of a 32 feet long, 26 feet wide and 14 feet tall room (the ceiling really has to get out of the way). Or whatever the length has to be at that width for a golden ratio.
For a really good room I'd consider a width of 20 feet the bare minimum, and preferably I would have it much wider, hence the dimensions.
I am very happy and pleased with the sound I have been able to achieve in my room of 24 ft long x 12 ft wide x 8.5 ft high, but it's just too small when it comes to qualifying for a great room.
The relatively low ceiling was a huge problem even with my speakers that are not large at all (they are 4 feet tall). I have found a good compromise with extensive ceiling treatment, but it's still a rather unwanted compromise.