If you are talking when using the walking foot, it's a personal choice, what size looks good to you. Sometimes I use 2.0 up to a 3.0. Don't usually go any bigger than that. If you are talking free motion quilting, you are in total control of the stitch size because you dropped the feed dogs, and they are what pull the fabric through and make the stitch size what you have set on your machine. So with free motion and no feed dogs, it's up to your speed of moving the fabric and the speed of the machine. Believe me, it's easier to keep your stitches more regular and looking good if you will sew faster. Always have a practice sandwich made up and let yourself warm up before starting on that special project. I've found that some days I do a good job at quilting and some days it's just better to not do it and go do some other job. I've also read that if you practice about 15 minutes a day you will be suprised how much better the learning curve will be.