I tried on every harness I could find for Katie Scarlett when we got her. Every single one of them were too big. I had to go into the pet section at Petco and get her a cat harness until she grew a bit bigger. This is the one we got:
Coastal Pet Pink Mesh Cat Harness at PETCO
And being a little over $5 it did not break the bank until I could get her a better one when she grew.