800' sq + some, all useable bar the stair-well. I atually find I rattle around in this huge space like a single pea in a barrel, even with the cat to keep me company, but it comes with my job, and is ex-British military.
From experience over the years, my preferred area is 300 - 400' sq. Doesn't take so long to clean, and if well organised has room for a civilised dinner party (6 - 8 people) and overnight guests.
The smallest flat I've lived in was 150' sq, which I shared with one other person (but there had been a family of 5 living there previously). That was a bit of a squeeze: the 5kg washing machine was mounted on the "bay window" ledge in the living room, we literally kept the wok hung outside the kitchen window, and I had to kneel on the toilet seat to hang the washing on the outside line.