Belgian roundels primarily intended for 1/300 scale aircraft.
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. Not that Belgium had much in the way of large aircraft at the time....) The actual diameters are 2.5mm, 3mm and 4mm.