The first step is to decide how big you want your pillow in length and in diameter. If you plan to make your own feather pillow filler/insert like I did, then you can make your bolster any size your heart desires. However, if you want make things easier, I would say order or buy your filler first, before you make your bolster cover, so you can use the dimensions of the filler.
I cut my end circles 9" in diameter so my finished ends would be 8" in diameter. I used a kitchen plate, however you could easily customize the size of your end circle using the thumbtack and string method.