What's the best needle size for injections?

[Papirus]

I'm very lean and I wanted to ask you the best needle size for injecting SubQ and the best size for IM, I currently have 6mm 31g for SubQ but I have to buy the syringes for the Test and Gluth what's the smallest size I can get to still reach IM?
Thanks

 

[DravidianBvll]

1 inch is the minimum for IM

 

[CertifiedGoy]

18g 3 inch