Plain red stars. These were actually relatively unusual on aircraft, but do of course have all manner of other uses as well. That said, I personally don't use this sheet very much; for most purposes I'd recommend 300-RU-3 instead, as the designs stand out better when applied to a model.
These decals are scaled to represent 30", 36" and 48" height markings in 1/300 scale. The actual dimensions are approximately 2.5mm, 3mm and 4mm. While primarily intended for 1/300 scale fighters, they're also ideal for heavy bombers in 1/600, and even for use on larger scale vehicles.