Only to give you a data point, in my country (Italy) norms require (since the '70's):
1) at least 9 sqm for a bedroom (for 1 person, 12 sqm for a double), rooms below 9 sqm in size can only be "studios" or "storage" or similar (at least officially)
2) at least 28 sqm total size to have the dignity of "flat" for 1 person or 38 sqm for 2 people (i.e. anything below 28 sqm cannot be registered as living space unit and while you can of course have two people living in a 28 sqm it is not "legal" , more generally, to be "legal", you need 28 sqm for the first person and additional 10 sqm for each other person resident in the house)
3) 2.70 m height for any room where people may stay for long periods of time (i.e. exception made for corridors, closets and bathrooms that must anyway be 2.40 m)
4) at least one bathroom of at least 4 sqm for each unit
5) a window or similar that must be at least 1/8th of the surface of the room (again excluded corridors, closets, bathrooms) for light and ventilation