1. Measure the door. On all sides, subtract an amount equal to about one-sixth the door's length or width. (For example, if your door is 24 inches wide, subtract 4 inches for the left side and 4 inches for the right side, and you have a width of 16 inches.) Have a mirror cut to the size of the smaller measurements. Or check at home improvement or housewares stores for a mirror within about an inch of the size you need.