This is very dependant on how full you would like your cushion pad to fill your cover.
If you order a 17"x17" cushion cover for example, we recommend an 18"x18" hollowfibre pad for this to make the cushion plump and fills out all the covers edges. However, if you would like a much smaller cushion and the edges of the cover not to be filled out, our 16"x16" hollowfibre pad will fit into this with ease and leave extra space.