It depends. The different modules of the NOVA bike light fits on different diameters.
The dynamo fits on a maximum of Ø 50 mm. / 1.96 inch. There is no minimum.
Front light for head tube:
If you buy a front light for the head tube you will receive two silicone rings; one small and one large. The small ring fits a diameter on Ø 30-40 mm., the large ring fits a diameter on Ø 40-65 mm.
Rear light for seat post:
The rear light comes with a silicone ring that fits a diameter of Ø 30-40 mm.