If you're going to use a sheet, PLEASE be sure to wash it first, to get all the shrinkage out of the way!
I like to economize by buying an extra twin-size sheet and cutting it down to make several pillowcases. Once I made the mistake of not shrinking the sheet first, before cutting the fabric for the pillowcases.
Then after the pillowcases were laundered the first time, they were no longer big enough to completely cover the pillow!
(Which also says that most of the shrinkage was along one direction, not both... otherwise I would never have been able to even get the cases back onto the pillow at all!)