Well it took me 2 minutes to open this, but here you go
use a 1 inch needle, unless you are very fat.
Also a 25 gauge is a must. This way you cannot put the stuff in too fast and prevent absesses;
Remember this:
1) the longer the needle, the more force is needed
2) the bigger the syringe, yep the more force is needed
3) the thinner the needle, the more force is needed
So use a 25 gauge and heat the syringe with hot (not boiling) water.
depends where you are putting it. Assuming the glutes, I would use 25 g and 1.5 inch long. The reason being that you should leave a little bit of needle part out of your injection site. That is just in case the needle breaks and you dont have to go to ER looking like an idiot saying you got a needle stuck up your butt