I'm not too sure, I've never actually kept chickens. But My Pet chicken (yes, most of my information relies on that site, and several books) says to have 2 sq ft per chick, because they're growing. "Living With Chickens" says about 1 sq ft. Have no idea, I'm sure people have kept chicks in smaller spaces before and they were happy the whole way along. Good'luckers!
Depends what they are... I can fit 2 dozen japanese bantam chicks in a 36x18" aquarium for about 4-6weeks but I bet I couldn't even put 5 standard chicks in there for that long. I'd probably lean towards the 2 sq ft for standards as a rough guide.
How old are your chicks? Temperature-wise, you're supposed to start at 95 degrees the first week, then drop 5 degrees each week till they're fully feathered (around 6-8 weeks I think). As for how much room - just keep in mind that if they feel too crowded they might start beating up on one another from stress - so more room is always a good thing. If they're still small little chickies you can get away with smaller space, but as they get bigger they of course need more room. How big is the space they're in right now?