For very specific instructions, it is helpful to know the size of the pillow that the pillowcase would be covering.
Here is a link to some bed pillow sizes:
https://casper.com/blog/pillow-sizes/
The ones illustrated in that link are all 20 inches (51 cm) by various lengths. The "regular" size one is 20 x 26 inches.
Generally speaking, if using fabric that is at least 42 inches wide, I would use the length of the pillow plus at least 5 inches more for a hem and seam allowances.