Depends on what your giving.
Sub Q (under skin) or IM would determine length. For Sub Q I usually use a 3/4 inch. For IM I use 1 inch.
Gauge (the first number and what determines the diameter of needle) highly depends on what your giving. If giving Alpha 7 an 18 gauge is almost too small, as the vaccine doesn't flow very good through it, so I usually use 16 gauge.
If it is a med/vacc that has the consistancy more like water, use a smaller gauge which means a larger number. I would use a 18
If vaccinating cows around a chute, I would use a 14 X 1. Remember, the lower the number the larger the needle. The higher the number, the smaller the number.
I never go larger than 14, or smaller than 18.
So in summary, determine HOW you will give the meds, and WHAT you are giving, you will then know what needle to use.