The pattern for this pillowcase was a free hand-out at Nancy's Quilt Shop, however, I felt that the instructions were difficult to understand so I Googled around and found this one, which is easier to understand. I found another site that gave measurements for both standard and king sized pillowcases. That one said that you need 29" x WOF (width of fabric) for a standard pillowcase and 39" x WOF for a king sized pillowcase.
The one I made today for my youngest son is standard sized and it is a very generously sized standard. I like that you can't see the pillow peeking out of the opening, like the one currently on his pillow. That just drives me nuts. I will definitely make more of these. I want to make my older son one with a football themed fabric, if I can find one I like. I think it will be great to take to camp or sleepovers so they can easily tell which pillows are theirs.