Plenty of them in Stockholm and Paris. I once shared a 20m^2 flat with my partner; it worked alright. Just can't accumulate a lot of stuff. To give you an idea, here are some for sale in Stockholm county right now:

https://www.hemnet.se/bostader?location_ids%5B%5D=17744&item...

Here's a 9.5m^2 (!) apartment for sale (including bed, work area, tiny kitchen, toilet, shower), although it's marketed as something in the middle between hotel room and apartment (that's one way to put it, I guess):

http://www.bjurfors.se/sv/tillsalu/stockholm/stockholm/bromm...