should be a division of the finished block size, to keep things balanced looking...such as 3" sashing strips on 12" blocks, 2" on 8-10" blocks, it really depends on the design, how big you want your quilt to grow, but they should be in relation to the block size...if you have a 12" block and put 5" sashing squares around it it will look...odd...try out different sizes to see what you like