A standard pillow case is 21x32 inches. A king is 21x36 or even 21x40.

41 is not wide enough if you are folding it selvedge to selvedge.

A lot of it depends on how much give you want on the pillow and how much extra material you are willing to fight while you're asleep. My newest pillow cases measure 29 1/2" by 20"; I've noticed some older ones I have are a half-inch wider and/or half-inch longer. Most of the ones I inherited are within 3/4" of the same measurements. to me it goes back to how much excess do you want to deal with when you're dressing a pillow or trying to reposition a pillow while you sleep.