How to adjust the object size in 3D only display area?

Hi,all.I am struggling with this task,I will be very grateful if anyone can give me some possible advice.
This question is more about vtk.
I have a cube(volume) and the information is as follows:

image dimensions * image spacing = 30,30,40(mm)
So I want to adjust the camera to make sure the size of volume is 30(mm)x30(mm)x40(mm) and also I use the P-A orientation and show the ruler as follows:

So my goal is to make sure the rectangle is 30mm by 40 mm in this case.
I can get the camera

view =
renderWindow = view.renderWindow()
renderers = renderWindow.GetRenderers()
renderer = renderers.GetItemAsObject(0)
camera = renderer.GetActiveCamera() 

Also,I have many parameters

angle = math.radians(camera.GetViewAngle())
point = camera.GetFocalPoint()
direction = camera.GetDirectionOfProjection()
scale = camera.GetParallelScale()
distance = scale / math.sin(.5 * angle)

I wonder if there is a way to calculate the width and height of the cube with above parameters and adjust the camera to make the size of the cube become the one I need.

Thank you in advance for your help!