The top of the clip is 11.4cm and the underneath curve is 6.4cm. It will adjust to open to 7.6cm and fit round a complex, sloped, thick, thin, shallow or brick mantelpiece. See image for various options. The listing is for one mantle clip.
The chalkboard measures 10cm x 5cm.
Please see pictures for diagrams and sizes so that you can check your mantelpiece carefully before ordering.