A 5' wide room is _really_ pushing the limits of what a space can be used for -- you can't even place furniture on both walls and that's less than 2' from the side of a twin bed to the wall. I mean sure, you can use it, but it's _extremely_ limiting.

Yes, a 5' (1.5m) wide room is more a corridor than a room.

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

A 10x10 room can’t be split into 5x10 rooms.. need some space for the wall in between.

Then you have two doors, and the space those doors need. Two windows for emergency exits.

Sure. But even if you handle the windows, doors, and have 3.5" for the wall (2x3 partition wall with half inch sheetrock) it's still i prohibited.