This is what I use and have always used may not be right according to the book.
Sub-Q (under skin) 18 gauge 3/4 to 1 inch
Intermuscular- (in muscle) 16 gauge 1 inch
recently the only muscular shots I've given are letulolayse. Most all the rest go under the skin for Beef Quality assurance.
If just working cows only I will often use a 16 gauge 1 inch for sub-Q shots as well
IV's--don't do many but I use a 16 or 14 gauge 1 1/2 inch for this. But have probally only given one about every 3 years. Knock on wood haven't had to give one for some time.