Finding the Ideal Wrist Coil

I’m more drawn to rope-based stuff. I struggle a bit with tying the ideal wrist coil, and when I do manage it, my knot tends to slip or break loose. Plus, that wrist coil is rarely sized just right. So what I really want to know: what’s a good size for a wrist coil, and which knot works best? Appreciate your help ahead of time =)