Depends on your skin vs belly fat thickness, and how deep is the happy place for you, but to get under skin, 5/16" is fine. I have even used 3/10" just fine straight in when I ran out of syringes and they were the only thing available.That said, I have mostly skin little fat. If I stick straight in with 1/2" I am in muscle.I don't want to get into the IM or Subq debate.
What size needles do most people use to inject Subq. My doc wants me to use 3/4 inch and inject at a 45 degree angle in my belly fat.