In my opinion, the guard size should be dicated by the peep size you have, plus the distance from your peep to the sight. I personally have a small guard and my Fletcher 3/16" peep lines up perfectly in my Hunter Hogg.

Pin size, is really a personal preference. The larger diameter fiber is stronger and will take a bit more abuse, but you sacrifce the pin point accuracy of a smaller pin. Also, the larger fiber gathers a bit more light than the smaller fibers. I actually switched from .029 to .019 for all my pins. I don't think I'd go any smaller as these work for me.