We use a 22 gauge for most things. They are very small, so good for thin skinned animals, like goats. We buy the single use needle/syringe combos from Jeffers, very convenient, and not very expensive at all.
We give EVERYTHING subcutaneously. Goats have a very fast metabolism, it gets where it's going just as fast as im, and it is a LOT less painful.
The only exception would be very young goats. Then we tend to do im, just to be on the safe side-baby goats tend to succomb to things MUCH more quickly than adults.
To do sub-q: find loose skin (just behind point of elbow works extremely well), pinch skin to make a little tent, insert needle, depress plunger, and voila! you gave a shot! If we are giving a large dose of something, we split the dosage and give one on each side, also, we stick the needle nearly all the way through and withdraw the needle while depressing the plunger-spreads the dose out a bit more so it's not as uncomfortable.
To do im: find a large muscle (we usually use the thigh, it's generally the biggest), insert your needle until you are sure you are well into the muscle, pull UP on the plunger, if you get blood, try again, if no blood, depress the plunger.
A lot of drugs that are perfectly safe injected sub-q or im are lethal if accidently injected into a vein. This is another reason we do sub-q (very extremely LOW possibility of hitting a vein), and the reason you need to double check when injecting IM.
Hope this was helpful!