You want to be about in the middle to deeper half of the muscle head. So if you think there's about 1 1/2" of muscle between the skin and the bone, go with 1". However, if there is 1/2" of fat, plus a larger muscle mass, you might want to go with 1.25 or 1.5". Or if you're a leaner type with no fat and smaller delts, you can go with a 5/8 or 3/4" needle. But with that said, if you don't have fairly thick delts, I'd suggest not injecting them. You'll likely have more soreness. In that case, go for glutes or outer quads. I use 1" on delts, and I have moderately big delts and no fat layer.
Charles
Usual disclaimer: I am not a doctor, and I am not giving medical advice! I am simply sharing personal experience based on my own life, which may be entirely wrong for anyone else.