I use 18 gauge needles, 1" most of the time. The only time I've used 20 gauge was on a very small cat who couldn't cope with the 18 though she also had a hard time with how long the fluids took to go into her. Perhaps they sell more of the 20 gauge because people are afraid of hurting their cat, or because in the later stages CRF cats become very thin and their skin becomes more sensitive.

Do you have a spare line on hand? I always have some spare ones now because there were a few occasions when I'd get distracted as I was putting the catheter into the bag and I'd hit the rim of the opening, contaminating the catheter.