I'm just being taught to inject, and the nurse started me on 1 inch needles all the way in. We tried 23 gauge but found that 22 gauge needles hurt less, but she only had 1.5 inch 22 gauge needles, so when she used that, she didn't go all the way in. 1 inch may not be a lot, but it sure feels like it goes into the muscle.
As for aspirating, she taught me to aspirate but said that the newer nurses aren't being taught to aspirate any more since it's so rare that you ever hit a vein. She said she's never done it in 25 years. But she still aspirates just to be safe.