Agreeing with the others, it's all about how much fat... Take a good pinch of the area you're going to stab, and see how thick the pinch is. Let's say it's 1" thick, then divide that in half since you're only going to stick the needle through half of the thickness you're pinching, and add at least 3/4" for muscle depth; meaning that the person with a 1" pinch, needs a minimum 1 1/4" needle just to get inside the muscle mass enough to be a true IM delivery. 1.5" is the way to go for most physically fit adults, and now a lot of medical articles are saying to use 2.5" on those with thicker than average fat layers. I have very low body fat, and I use 1" for delts and quads, but still use 1.5 in the glutes.
Charles