I lived for a few years in 21m². It was nice, very central Helsinki, well thought-out layout. When I wasn't living alone, we upgraded to 30m². Even better layout, in multiple levels. In effect you had a separated bedroom, living room, dining room, and bathroom.

In Berlin we had 60m², which was a ridiculous amount of space. Enough to host multiple guests.

Now we live on a 9.3m long sailboat. Square meters are a bit difficult to calculate. Maybe 16m² for the interior space?