In Librealsense, the focal length is processed within the rs_intrinsics struct. The fx and fy fields describe the focal length of the image, as a multiple of pixel width and height
In the script in the link below, an example of the use of rs_intrinsics to calculate focal length can be seen in lines 300 to 310 of the script.
Line 597 of the script is used to retrieve the details of the intrinsics with the 'rs_get_stream_intrinsics' instruction for any stream which has been enabled with a call to rs_enable_stream or rs_enable_stream_preset.
Full details at this link:
I would assume that fx and fy are the horizontal (X) and vertical (Y) length of the focal (F). Hence fx and fy. So on the diagram on the site you linked to, 'focal length of the height' should be fy and 'focal length of the width' should be fx.
In the formulas on that site, object height and the distance between the camera and object are the easiest values to work out.
This image from the R200 data sheet describes the length and width of its field of view for the Color and IR cameras.
Ok, I understand, and the unit of fx and fy is mm, right?
In the web-page about projection that I linked you to, it stated that the default measurement scale for the R200 is mm. I also generally find that mm is the appropriate scale to use with RealSense functions. Also, with real-world cameras the standard measurement scale for focal length is mm.