I use strings between 1 and 2.5 inches mostly unless it is going to be a big quilt then I go up to 3 inches. I have put as narrow as 3/4 an inch sting in but you do not want to put too any of those because having more seams than top fabric would make for a very heavy quilt as well as take a really long time and lots of thread to piece. I do love the look however of that little 1/4 inch color pop between two more subtle fabrics.