First off, 1.5 is the length -- the needle is 1 and 1/2 inches long. And you are right, the higher the number the smaller the pin (in diamater). A 23 ga 1.5 pin is good for most glute and quad shots. Personally, I like a 25 ga, 1 inch for quads, 25 ga, 1.5 for glutes, and 25 ga, 5/8 for...