Scott & White Healthcare states that a needle gauge between 22 and 25 is ideal for an intramuscular injection. The needle length varies depending on the size and age of the individual receiving the injection, but gauge does not matter.
According to AboutKidsHealth, smaller gauge needles are larger in diameter. Larger needles may seem more painful, but because it is easy to push medication through a thick needle, the injection does not last long. AboutKidsHealth also states that topical anesthetics, such as lidocaine, tetracaine and EMLA, help with injection pain, which is especially useful for children who are nervous about needles.