As far as I know camera objects don't have geometry, and hence no real world Blender Units dimensions.
The viewport size of a camera object depends on a variety of factors.
The camera object size reflects the render resolution set in the render properties panel, changing proportions according to the image size. On top of that a scaling factor is applied, controlled by the Size property in the Object Data panel > Display > Size.
Besides that the Focal length will also affect the camera shape by adjusting the angle of the sides to reflect the current view field.
You may be able to calculate the resulting viewport size by recreating the math internally used by Blender to represent said camera object though, but I wouldn't expect that information would be readily accessible from the Python API.