When choosing the size of box for your fan, allow between
5 to 6 cms. clearance between the inside of the frame and
the periphery of the fan.  An example of how to order the
correct size of box is as follows -
Measure the guard stick from pivot to outer edge i.e. 18.5cms.
Multiply by 2  =37cms.
This means ,the fan when fully open measures 37cms wide
Allow 6cms each side for clearance = 49cms.  (37 + 12)
The nearest width of box  to 49cms is 50cms.

The box most suited for your fan is 30cms. high x 50cms.wide