Pillowcases are simply fabric covers for pillows that protect them from grime and stains. Washing a pillowcase is much easier than washing a pillow and, unlike pillows, they can be laundered dozens of times. While you could put a king size pillowcase on a standard size pillow, using the correct size will look better and be more comfortable.
A standard pillow is 20 x 26 inches; a queen size pillow is 20 x 30 inches; and a king size pillow is 20 x 36 inches.
Pillowcases are slightly larger than pillows to get them on and off easily. The end that opens also extends some 6 inches longer than the pillow, so the pillow is not exposed when it is fluffed or slept on. The finished size of a standard case is 21 x 32 inches; a queen size pillowcase is 21 x 36 inches; and a king size pillowcase is 21 x 42 inches.
Fabric for Pillowcases
Pillowcases are relatively easy to make. They are essentially two rectangles of fabric sewn together on three sides. You will need 2 yards of 44-inch wide fabric for a standard size pillowcase, 2 1/4 yards of 44-inch wide fabric for a queen size pillowcase and 2 1/2 yards of 44-inch fabric for a king size one. Perhaps the least expensive way to obtain this fabric and to ensure it matches, would be to buy one twin-size flat sheet to use for pillowcases when you purchase the sheets for the bed. The fabric you will use for a pillowcase is most likely to be a cotton or cotton blend, so preshrink the fabric before you start.
You can add decorative touches to a purchased or a handmade standard pillowcase. Some suggestions are to add a border of contrasting fabric at the open end of the pillowcase, sew rickrack, piping or lace to the upper edge of the pillowcase or embroider a design on the face of the pillowcase.