Thank you for reaching out to us. I am not sure if I do understand the question correctly.
I see that the camera gives me values in degrees, but I need to have them in CM
With the possibility of stating here the obvious, let me spell out some things, so that we are on the same page. The dimensions of the view frame are usually measured in pixels, i.e., the dimensions of the photo or rendered image. You can convert that into a measure of distance, e.g., cm, but for that you need to define a pixel density, the DPI. The rest is just multiplication when you know how many pixels should fit into an inch.
The pyramid shown in the viewport is just a visual aid to represent the camera frustrum. There is normally no physical quality to it. When you want to calculate the physical dimensions of the view frame, as if it would be projected onto the plane that contains the focal point and is normal to the camera z-axis, i.e., the flat plane which is in front of the frustrum gizmo, then this is just an exercise in quite simple trigonometry. There is no method in our API which would do that for you, as this is a rather uncommon thing to calculate I would say. And this would usually be a value in meters and not centimeters for the common Cinema 4D scene, as the default value of the focus distance of a C4D camera is 2m.
So, when we know the length of the focal point vector, the adjacent (b) in the triangle, and we know two angles, the horizontal and vertical field of view, then we can calculate the opposite (a), i.e., what you seem to be after.
The opposite and the adjacent are defined over the tangent of the input angle:
You then just must remember that you must cut the viewing angle in half, since we only calculate a subsection of it and then multiply the result of the opposite (a) times two, since you are after the full length. You then must do that for both the horizontal and vertical section and it will give you the length of x and y in your image.