I honestly never really measure my length. I piece whatever I have and think would look nice. I am sure that I have used 10" lengths. I don't know if I would go much smaller than 10" for a bed size quilt
if you are making a scrappy binding for a table runner I would probably figure using strips about 1/2 length of the side. make sense? like if the the table runner is 20" x 40" I think 10" would be fine
just my opinion though Do what ever you think looks good