For IM injections, if you have zero body fat, you can probably get away with 3/4 or 5/8 needles, but the fact is that with only a 1/2" insulin needle, you're so close to the muscle sheath that it might not get all the way into the muscle tissue. Personally, I use 21g 1" for delts, and 19g 1.5" for glutes. I'm at 5.3% bf, and that seems to mean that my muscle tissue starts about 1/8" below the skin on the delts, and about 1/2" on the glutes. But I don't think there's any rule about it. It's all preference, as long as you get the injectible all the way into the muscle tissue beyond the muscle sheath.
Only thing I need to add, is that if you're trying to conceal needle marks, you are stuck with thin-gauge needles (pun intended), but still need to have them long enough. I have plenty of visible needle marks on both delts and glutes, but I'm never going to be in competitive sports or in any bodybuilding contests.
Charles