I measured my head exactly according to the instructions and ordered size M, as my head circumference was 55 cm.
When the helmet arrived, I wasn’t able to try it on, and only a month later, when I returned home, I finally tried it and discovered it was too small.
I asked the company to exchange it for a different size, but they refused because more than 30 days had passed.
I explained that I couldn’t try the helmet earlier and that it has never been used, but they still denied my request.
Now I am left with an expensive helmet that is too small for me.