Hi, I try to use the smallest possible, like a 22 or 24gage.(the larger the number-the smaller it is) If it is a small chicken or chick I will use a diabetic needle, although it takes awhile to get the meds out in this size.
1) fill syringe with amount you are to use, depressing all the air out
2) lay chicken on it's side, and either with someone holding it's feet or you holding it's feet together in one hand, and using some rubbing alcohol etc. to wet down and move to the side the feathers on the breast area.
3) Find the thick part of the breast, then find the center most area with most flesh.
4) inject needle in to fleshy area, just enough at an angle to go under skin and just into flesh.
5) pull back on syringe to check for blood, if no blood inject the med.