It should be around a yard to a yard and a half.
King
A pillowcase is a covering that you slip over your pillow/s that matches your sheets/bed linens. It is rectantular in shape and open on one of the narrow sides to slip the pillow in/out. In the US, there are three sizes: the small size if called Standard; the medium size is called Queen; and the large size is called King.
size of king pillowcases
One can purchase a hypo-allergenic queen size mattress cover from the following sources: Marks & Spencer, Bed Star Direct, Duvet and Pillowcase Warehouse, Amazon, Bed Bath and Beyond.
A comforter set usually includes a quilt (comforter) and a pillowcase or two (depending on the size of the set, queen or king). Sheets are usually not included.
as big as a single bed
When laid flat, a king pillowcase typically measures 20-by-36 inches or 20-by-40 inches. It is typically the largest size pillowcase offered.
King.
Standard pillowcases are all the same size; about 40cm by 80cm (16" by 32").
Pretty small
They all vary to the inch, but the 'Queen' size bed is the largest of the Bed Family.
a pillowcase