When I repotted my fiddle leaf about a year ago, I went for a new pot that was just 2 inches larger in diameter. It seemed to give enough extra room for the roots without overwhelming the plant. I used a well-draining mix—one part regular potting soil, one part perlite, and a little bark—for a little extra airflow. A tip that really helped was watering the plant a bit before repotting so the root ball stayed intact, and then keeping it in a bit of shade for a couple of weeks afterward. Hope that helps!