So, with these data in mind, I think it's safe to assume that a 1-inch needle is sufficient in all types of men for deltoid injections and the same can likely be said for injections in to the thigh. As for the buttocks, I'd say the safest bet is to go with a 1 1/2 inch needle. However, there are going to be plenty of guys who are lean enough to use a 1-inch in the buttocks and a 5/8 inch in the deltoid, and even a 1/2 inch needle in the thigh. To be 100% sure, however, use the lengths suggested above.