I purchase a syringe with a 20g pin attached and use this for drawing up my oil from the vials. Then I switch to a 23g pin for injecting. The length depends on the muscle that I am using. For glutes, use a 1.5in needle. For quads or delts, 1 in. For more info, here is a write up that juiced muscle has on the topic :
http://juicedmuscle.com/showthread.php?49-A-Guide-to-Safe-Injection-Technique