The pillowcases can be of different sizes and thicknesses. And depending on the country, the sizes vary a lot because we have different customs.
If you have a bed, for example; of 150cm (of width), the normal thing is that the pillows add in its longer part 150cm, so you need two pillows of 75cm long and as for the width; particularly, I like them wide, at least of 45cm. Do not buy a 40cm pillow that will make your bed small.