Afraid I don't have any good rules to follow. I usually fiddle around with scraps of border fabric, batting, and backing until I am satisfied with the look and tension. Obviously a design with a lot of tight turns works better with a smaller stitch and you can use a larger stitch on something like clamshells.
I typically sew with a tiny stitch and needle, but when I quilt I bump up both the needle and the stitch size. I usually quilt with a 14 needle of some sort and depending on what I am doing/the fabric/batting I am using, will go up from there.
Another variable is your batting. I typically use a fluffier poly batt with (a lot) less quilting than is currently popular. Is much different using Warm and Natural or something like that or thinner.