I would suggest a 25g 1" pin for bicep injections. You want to make sure that you get deep enough into the muscle to properly stretch the muscle fascia.
I think even a 5/8 X 25-28G can work well for most guys, as long as they are pretty lean. However, as the others have said: better safe than sorry. I would go with a 1 inch X 25-28G to err on the side of "safety" where getting deep enough into the belly is concerned.