Hi Thomas,
I recall that you are working with Unity. In that case you can use Screen.currentResolution to get the screen size in pixels.
http://docs.unity3d.com/Documentation/ScriptReference/Screen-currentResolution.html
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.