I'd measure the pillows for the cases. I absolutely HATE the cases that come with sheet sets, they are so skimpy even with the more expensive brands. I hate the way I have to force them together to shove them into the case when the bed has to be remade.
I make mine about 3 inches wider and 4 longer than the ones I buy. When I have to change the bed, I don't wind up mad at everything now.
Each time I buy a set for the bed, I buy an extra bottom sheet just for this, and use the extras for quilting.