A standard pillowcase measures 20-by-26 inches, which is too short to fit a king pillow and too short for a queen pillow. A queen size pillowcase is typically 20-by-30 inches. Queen-size pillows are sized to fit two comfortably side by side while extending most of the way across a queen-size bed measuring 60 inches wide by 80 inches long.
Assuming your friends are adults, I would make it a queen size pillow case 20 x 30. It would fit on all sized beds. Most Adults in US use queen size beds, and even those that use king sized beds, most often use queen sized pillows. If your friends are children, they are probably in a twin or a double. Again, a queen sized pillow looks best on those beds and they probably have queen pillows. And if they have regular pillows, a queen case will fit it without looking too big.
Your other choice is to make it King sized and let them sew the one seam to make it shorter, assuming they have the skill and the interest in modifying a gift. And Kings cases never look good on a queen or regular pillow.
Again, I would make it Queen Sized 20 inches by 30 inches.
Hope this helps you