Ok, My sister said that she isn't familiar with BP injections but she did explain to me the basics.
Obviously you need a syringe that is big enough to hold the full dose of Baytril.
She then said that most needles used are in the 20-22 G (gauge) but the length differs. (needles will be labeled like this: a number, a G, then another number. The first number is the gauge of the needle and the second number is the length)
I know that I didn't fully answer your question but I hope that this helps