Oh yeah - if you want good and well stuffed pillows, you always want to make your pillow cover at least a 1/2" smaller than your pillow form.
So if you have a 20" x 20" pillow form, make the final dimensions of your cover 19.5" x 19.5" (at most). I often go down a whole inch if the pillow form is flatter.
Martha Stewart online has a great quick pattern online adaptable for any size and with 3 closure options. I can't link on my phone but if you Google you'll find it. I like an envelope closure with added buttons since it's easy to remove and wash and then the buttons can be an added decorative element.
I've made plenty if envelope pillow cases for pillow forms. I take the easy route. I measure the pillow form, add .5 inch extra to both measurements. Cut and see leaving .25 inch on each side. I don't love sewing button holes or zippers.