Six of them, each with its arithmetic
Every one of these is a common rule of thumb out of decorating practice. None of them is a standard and none is a requirement. What each page adds is the arithmetic underneath, so the rule stops being something to obey and becomes something to check.
Area Rugs & Runners
A common rule of thumb leaves roughly 18 in of bare floor between the rug and the wall on the main sides of the room, and puts at least the front legs of every seating piece on the rug.
10 of 10 listings state 36 to 120 in
Curtain Panels
A common rule of thumb hangs the rod close to the ceiling, buys the panel long enough to reach the floor from there, and puts about twice as much flat fabric across the window as the width being covered.
10 of 10 listings state 63 to 84 in
Floor & Full-Length Mirrors
A common rule of thumb is that a flat mirror shows you a stretch of yourself about twice its own height, so glass roughly half your height can cover you — but only if it is positioned so that stretch lands where you are.
6 of 7 listings state 56 to 65 in
Floating Shelves
A common rule of thumb spaces shelves by the height of the tallest thing that will stand on them plus about 2 in of clearance, and hangs the lowest shelf about 8 to 10 in above whatever furniture sits below it.
7 of 8 listings state 15 to 16.5 in
Floor Lamps
A common rule of thumb puts the bottom of the shade at or just below the seated eye height of the person using it, so the bulb is hidden and the light falls on what they are holding.
7 of 9 listings state 60 to 72 in
Night-Lights
A common rule of thumb places a night-light low, near the floor it is meant to show, and picks the lowest output that still makes the floor readable rather than the brightest one available.
No listing here states a figure
The measurements these rules need: Measure. The shapes of room they land differently in: Rooms.