The smaller the pin will help if your shooting longer yardage. It won't cover up as much of your target down range. I like the smaller because of this. I like to practice out to 80 yards. If I use a larger pin I would be guessing on aiming most of the time.
LCPL Ralph Fabbri 9/29/89-9/28/10 KIA fighting for our freedom.
I generally run 0.019" for my 20, 30, & 40 pins and 0.010" for the 50 yard pin. The smaller pin blocks out the spot I am aiming at less at the farther range. I only target shoot out that far and the bigger pins can totally cover a spot.