So how big? How big the geographic catchment for most members -- don't want to get caught in too parochial a concept -- and how big the meeting space?
First. So how big should the geographic bounds of the new church be? Conventional wisdom -- I forget the provenance -- suggests people will go as far to church as they will go to work, so Census records of commuting patterns would be helpful.
Consider Washington, D.C. workers. In this 2008 report of commuting patterns, 35.7% used transit -- indeed, 23.8% of workers own no car; I don't -- and 43.6% of commuters take 24 minutes or less to get to work. This suggests to me that some will drive but many will take transit or walk, and you can expect to get people to come in about a half-hour radius. Unfortunately, I don't have a good tool to suggest -- say -- a half hour's reach from point X. Perhaps there's some folk mapping tool out there. (I attended this workshop.)
I say this more to suggest that existing churches very often overstate how large their influence is, and miss out on opportunities for growth because they have to concentrate all their efforts in one building, when renting multiple sites -- even for single occasional uses -- would be more effective. Or alternately, be a clue that it's high time to start a new church.
Back to rules of thumb -- now for rental and purchased space -- and I'll save some thoughts for later.
11 sq. feet per adult for the meeting hall. The only really helpful and practical guide I found for this come from -- of all places -- the United States Air Force and its Religious Facilities Design Guide (PDF).