I would use a 23 gauge 1 inch needle. Dont go less than 1 inch because you want it to go deep enough that you dont risk an abcess. Using a 25 gauge that injection is going to take a hell of a long time to do, and in your delt is a bit hard to do and after pushing that plunger for 1 minute youre gonna get tired,lol! If you do decide to go 25 gauge, make sure you get a larger gauge needle to draw with or its going to take a long time to even get the syringe full.