The documentation for SetViewAngle() in vtkCamera says to use this view angle:
angle = 2*atan((h/2)/d)
Here h is the height of the object and d is the distance to the camera. For this to work with VTK, you must express the angle in degrees (not radians).
If you are using a parallel projection (camera->ParallelProjectionOn()) then instead of using SetViewAngle() you must use SetParallelScale(h/2).