To get started click the show head button in the bottom right corner.
Import your .stl file with the browse button near the top left.
You may need to zoom in or out a little to show everything on your screen. Scroll on this box or the footer to scroll the page.
Hover over the head measurements for info on measuring. This tool uses width, depth, and height.
In the 3D view right click to rotate the helmet, left click to move, use the scroll wheel to zoom.