This birdie I found in golden town Kremnica from middle of Slovakia. It was on the window on garden house.
You could choose form three sizes:
size 51 ‒ circumference 51 mm ‒ diameter 16.2 mm
size 53 ‒ circumference 53 mm ‒ diameter 16.9 mm
size 55 ‒ circumference 55 mm ‒ diameter 17.5 mm