If you are trying to display the object's elevation within the 2D script of that 3D object, SYMB_POS_Z is what you might want to look at. SYMB_Z_SIZE is only available for listing and labels (Label Subtype).
But, the thing is, SYMB_POS_Z only gives off the value that relative to the project origin. If you want to display ie. elevation/height from the current story, you need to request "Story_info" and do some calculation from there. There's an example for requesting story information in the GDL Reference Guide. Search for "Story_info" and you'll see.