you don't want the sashing to (over-power) the quilt blocks- there are no rules---but a fraction of the block size does help it look 'balanced' I generally decide by how much larger I want the quilt to be- then figure from there- if a 12" block I generally use a size that will (go into) a 12" block equally---such as 2", 3", 4", 6"...if a 9" block then 3" or 4 1/2" ; if a 6" block then 2" or 3"...see what I mean?...but just lay out folded fabrics in different widths, step back & see what you like- that's the size to go with.