GOOD QUESTION Everyone here seems to have a different opinion but the general consensus is between 23-25g and 1-1.5" long the length depends on your bodyfat and again preference. With Glutes I like 1" for Quads and delts 5'8" but that's me. Also I go 25g on all pins. here is what I do I Buy 3cc Luer lock syringes with 20g Pins and another box of 25g 1" pins only, then you can load the syringe fast and get straight to it. remember you don't have to put it in all the way so the 1" will cover you for all areas.