I would love to find out a way to know how much space I actually use. I expect it's not very much - while I say I use the living room all the time, really, I sit on one end of the sofa and that's it. The boyfriend and I are half-heartedly looking for a place to move into together, but a) we tend to disagree on how much space we need, and b) it's actually hard to find something that has everything we want and isn't 2000+ sq. ft. Everything that was built in the last 20 years seems to be huge.
I have a 1000 sq. ft 3 BR/1 BA, plus a storage/laundry room not counted in that as it's in a separate small structure. The guest bedroom is not used unless there are guests (so, maybe once a year), and I really only use half of the smallest bedroom/office, which is where I keep the ironing board set up. That could easily be set up in the guest bedroom or living room though, and the office-y part could be cleaned out. But I do use the kitchen, dining area, living area, bedroom, bathroom and half the office.
The boyfriend also has a 1000 sq. ft. place, but his is a 2/2 plus a garage. Again, the guest bedroom tends not to be used unless there are guests but we do use the second bathroom fairly often. His place has a breakfast nook in the kitchen that we don't use - it's too small to be comfortable and there's also a place for a dining table in the living area. We definitely don't need two places to eat that are so close to one another!
Based on our current space usage, I think we'd be very happy in a 750-800 sq. ft. 2/2 place. He thinks we need more room as we'll need separate areas to be away from each other (we are old and set in our ways, so moving in together could be a bit of a shock, plus I have 3 cats). I actually think a one bedroom place would be perfect, as hosting guests is a bit of a bore. But anyway, it all depends on what we can find.