Nozzle size is the diameter of an airbrush’s spray aperture, measured in millimeters: 0.2, 0.3, 0.35, 0.5 are the common steps. Smaller nozzles draw finer lines but demand thinner paint and more cleaning discipline; larger nozzles move volume and forgive thick paint.
For kits and miniatures, 0.3–0.35 mm is the honest default — fine enough for camo and highlights, wide enough for primer. Buy 0.2 for hairline detail later, not first.
See it in context in our airbrushes guides, or explore the tools hands-on in The Bench.