Originally Posted by Peckish

Are you asking what size needle you should use for quilting? Or binding? Because it sounds like you're already quilting it. If you're asking about binding, I would suggest that you don't stitch through all 4 layers. When sew on the binding, use a running stitch and go through the binding, top layer of quilt, and into the batting, then come back up. Don't sew all the way through to the backing. Then once you flip the binding to the back side, do the same thing - sew through the backing and into the batting, then back up into the binding.
Clear as mud?

There *are* needles that are marketed as binding needles, but that doesn't mean you have to use them. I've tried them, but because I've used betweens for so long, I find the binding needles too big and thick.