I'm glad you have yourself organized to take care of Max. I got some 1-1/2" needles, 18 gauge, once and used only a few because I found it impossible to push it in only two-thirds of the way when the cat didn't want to have kind of needle in him. They are now gathering dust and I'm happy with 1". With a longer needle there is also a greater chance of it coming out the other side of a skin tent. Perhaps the 20 will be best for Max.

Now organize yourself into looking after yourself! Remember, if you get run down you can't care for Max as well as when you are eating and resting enough. Care for yourself for Max's sake.