Bed Skirt Lengths. To get the drop size that you need to measure from the floor to the top of the box frame or bottom of the mattress. That will be the drop size you need for a bed skirt to touch the floor.
Bed skirt lengths can go up to 80 inches and the width of a bed skirt is dependent on mattress size. Measure from the top of your box spring at a point approximately 6 inches in from the mattress corners. Traditionally, it is long enough to touch the floor.