Part of it is determined by how big you want the finished product to be.
Depending on the pattern within the block, or a color that you want to accent throughout the rest of the quilt, you could use the measurement of a piece to determine sashing ...
If you consider that a sashing is actually an embellishment, whatever it is you are trying to accomplish by using sashing, that would be what dictates the size.
Try it with 2.5" sashing and see what you think ... <wave>