The model bounding box and radius (in the Header tab) are by default calculated, I think, based on all submodels (possibly including shield mesh), but it doesn't get recalculated when you import extra submodels, so in those cases you probably need to just tweak the values by hand or do a save/load (and possibly use the reset buttons next to the bbox and radius after that?) and hope that doesn't mess anything up.
Also note that the targeting brackets are drawn around Detail-0, so I think you usually want it to have the same bbox values.
I often have submodels extending outside the Detail-0 bbox, in which case my usual workflow is to either tweak the values by hand for Detail-0 and just check by eye that it's about right and then copy the values over to the Header tab, or to actually do the simple arithmetic based on the submodels' offset and bbox values to get the exact model-wide bbox values.