When choosing your needle size, consider the thickness of the stuff you are injecting, rather than the size of the animal. So for something thick you will need a white, or perhaps pink needle.
If you fill a syringe with water and try various sizes of needle, you will see that the finer the needle, the greater the force of the liquid coming out. As you can imagine, you can cause some tissue damage at the injection site if you use a fine needle and depress the plunger too fast. So you are better to use a slightly larger needle. I do use green needles occasionally for lambs, esp for subcutaneous injections, but I inject slowly.
For subcutaneous injections, use a 1/2" needle, for intramuscular, use a 1 1/2" needle for adults or 1" for lambs.
For sites of injection, everyone has their favourite. Wherever you inject, make sure you have avoided a major blood vessel (by drawing back the plunger before injecting to see if blood fills the syringe - if it does try somewhere else). You must also avoid the major nerves, so be precise when you put the needle in.
To stop the sheep from noticing the needle, tap the spot a few times with your finger before injecting - they don't seem to notice the final sharp one.