It should be pleasing to YOUR eye. I have been told that sashing sizes should be even divisors of the finished block size. Frankly, I don't set much store by that "rule", but often do follow it. For example, 8" blocks 'should' have 1 or 2" (finished) sashing. For a 12" block, 1", 2", 3" or even 4". 4" is just too big for most purposes. Tip: put your intended sashing fabric so it sticks out from behind a block. Slide the block around until what you have works to your eye. Once in a while I do pieced sashings. Those are an entirely different situation. The pattern needs to work with the block pattern. Storm at Sea is an example of this type.