As a general rule, a polyfill insert shouldn’t be more than about one inch (~2,5cm) larger than the pillow cover. For smaller covers (14”x14”(~36x36cm) or less) the insert should ideally be no larger than the cover. If you are using feather/down inserts, you may be able to use inserts that measure as much as 2 or three inches bigger than the cover depending of the ‘loft’ or volume of the insert. Source
When I sized my duvet from the cover I made it 3 cm smaller than the cover.