For obvious reasons, the RAF in the Far East rapidly decided that having big red circles all over their aircraft wasn't such a great idea.... Accordingly, by early 1942 most aircraft had the traditional RAF roundels replaced with a plain blue and white design, reducing the chance of mis-identification in combat.
These decals are scaled to represent 30", 36" and 48" roundels in 1/300 scale. (They're also ideal for large aircraft in 1/600; equivalent to 60", 72" and 96" diameter.) The actual sizes are 2.5mm, 3mm and 4mm.