I always set a slightly longer stitch length than I would normally use for sewing. My machine measures stitches with 5 being the longest and 1 the shortest, I use about 3 for stitch-in-the-ditch quilting and topstitching. Depends on the thickness of what I'm sewing.
Here's a website about stitch length.
http://www.dummies.com/how-to/conten...k-for-you.html