25g 1in works just fine anywhere on myself. Takes a solid minute to depress - I don't mind that. Pain is non-existent.
I'd say it depends on your body fat as to how long you need it. If you are very lean 1in (I know many who use smaller). If your carrying more body fat your gonna need a longer one. Just make sure your deep in the muscle.
The gauge will determine how long it takes to get it all in. I personally feel slower is better anyway. Gives the oil more time to disperse and makes the pain easier the following days.
If your a really lean guy I'd say 5/8th would do the trick on the delts... but why chance it, it shouldn't hurt once you break the skin anyway... whats another 3/8th for peace of mind?
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.