Minimum Requirements
Name Quantity Note Bed 1 Must be 5-star. Can be the single or double bed. Bathtub 1 Bedside table 1 Door 1 Fireplace 1 Light source 1 Can be any of the lights. Huge wardrobe 1 Must be 5-star. Size (squares) 4 Does not have to be 2x2, you can also do 1x4. Window 2 Does not have to be facing outside. Can face neighbours room :)
Do not place any lesser version of the Bed or Wardrobe in your room or you will lose stars.
i.e. if you have both a 5-star and 4-star bed, it will only count the 4-star bed.
Here's an example showing the bare minimum:
Optimizing
With the bare minimum done, let's see how to be a little more efficient.
The max amount of beds in each room is 5.
Here's an example of a 6 square room in a 2x3 configuration:
Notice how there's 4 double and 1 single bed. You can fit 5 double in a 7 square room, if you want.
However, that 7th square is better spent on another room.
Source: https://steamcommunity.com/sharedfiles/filedetails/?id=2656692620