To ensure that panels look ample and drapey when closed, they should have a combined width that is 2 to 2½ times the width of the window. Exceptions: If you're hanging curtains just to frame a window and don't intend to shut them, you can round down to 1½ times the width.
How to Measure a Mattress for Bedding:
Start with a bare mattress. Using the tape, measure from the left side to the right side of the mattress to determine mattress width. Next, measure the mattress thickness from the top of the mattress to the bottom of the mattress. This is the mattress height. Multiply the mattress height by two and add that amount to the mattress width. This amount will determine the width of bedding that you need to your mattress.
How to Measure a Mattress for Bed Skirt Drops:
Use a measuring tape to measure the distance from the top of the box spring to the floor. Add an inch or two to the drop measurement (the length of the visible bed skirt material) to ensure you purchase the correct size. An average bed skirt has a drop of around 14 inches, but longer bed skirts are available.