I recall that you are working with Unity. In that case you can use Screen.currentResolution to get the screen size in pixels.
The EyePositionData is indeed in millimeters, with the origin in the center of the screen, Z axis facing outwards.
Thank you for your feedback regarding API docs. We have thought about using Doxygen or some other tool to auto-generate the docs from the C API header files, but have not prioritized it yet. Add it as a Feature Request and we’ll see how many others who want it urgently.