Of course you cannot get the geometry of a family type. A family type is an abstract concept, basically just a collection of dimension values. The only concrete object owning geometry is the family instance inserted into a project.
Within the family document, you can determine the bounding box by iterating over all the elements inside the family definition, which might be solids of various kinds, cylinders, extrusions, etc., and summing up all their respective bounding boxes.
For an example of how that can easily be done, take a look at the ExpandToContain method in