SpideyPlanet@spidermabz For size M your head needs to be somewhere between 9 and 9.5 inch (23 and 24 cm)
If your head is less tall, then you can take size S. Be sure to check the image attached with the sizing guidelines.
SpideyPlanet@egcatalan730 Hey, thanks for your message. If you have any questions relating the costume you can contact Fernando on email@example.com. For questions that involve your shapeways order you can contact shapeways directly. Good luck!