You can make the sashing any size you want, as there are no quilt police! A rule of thumb I have read would be to make it 1/4 to 1/3 the size of your block. So if you have a 12 inch block, 3 or 4 inches. Having said that, I've seen quilts with 12 inch blocks and 1/2" sashing that I liked a lot!
How big are your blocks?