Single human with single cat. Layout is vital. If the layout works, a small place can be perfect. If the layout is bad, it can be much larger and feel too small.
My first 1BR (~600 sq. ft.) had a galley kitchen, separate dining area, and the bathroom was attached to the bedroom. There was nowhere to hide a mess, guests couldn't hang out in the kitchen, and the dining room was next to useless. (I did a couple furniture-finishing projects there.)
My last apartment was 70 feet larger, but the kitchen was open to the (large) living space, and you could get to the bathroom without going into the bedroom. If I were still renting, I would still be there.
My new place (2br, 890 sq. ft.) has an extra half-bath, a spare room, and more storage than I'll ever need (probably). There's also that much more to clean and heat/cool. Space-wise, it's much better than the other townhouse I've lived in (800 sq. ft.), and a small improvement over the 670-sq. ft. place. (Except that my new living room is the size of a postage stamp. I'm adjusting.)