Room Planning Guides
Furniture sizes, the space to leave around them, and how to measure a room, in centimetres and inches.
Almost every "it does not fit" comes down to a number nobody wrote down: the doorway, the depth of the sofa, the room a chair needs to pull out. These guides collect the numbers worth knowing, as common rules of thumb rather than laws, and say how to check them in your own room.
Each one ends with how to try it in Scooch, the iPhone app I make for exactly this. Where a tape measure and a pencil are enough, the guide says so.