Needle size. Good point on the length of the needle. I have seen offices use the 5/8" needle which is way too short to get into the muscle. You must use at least a 1" needle depending on the person you are injecting.
Into muscle. The trivalent inactivated vaccine for flu (flu shot) is injected into a muscle using a 22-25 gage (thin) needle 1 to 1 1/2 inch long. Site selection varies but is usually outer arm, deltoid region for older kids & adults, thigh in infants.