Hopefully 8k sqft was a typo. I feel like 2500sqft is too much sometimes.
With one kid or wife per bedroom and 1000sqft for kitchen/dining/whatever, you have room for 28. (4 wives, 6 kids each) Making the non-bedroom part of the house more luxurious and adding a bathroom to each bedroom, you have room for about 16. You can hit that even without polygamy.
He may be counting a large garage. That could be 1000sqft by itself, so knock off 3 or 4 bedrooms.
Fundamentalist Mormons are still Mormon, and yes, 10,000 to 30,000 of them are in plural marriages.
Also, note that I didn't mean it as an insult, and I quite like large families. Even the mainstream Mormons tend to have large families.