Rotation of model consisting of several meshes

If I want to rotate a model consisting of several meshes I know I can do that by selecting whole model (pressing a) then rotate it. But is there a way of doing this by using the rotate feature in the “transform properties” dialog (pressing n). When I try this I only rotate one part of the model. Somehow when I’m in the “transform properties” dialog I’m not able to select the whole model.

Parent all the objects to a central ‘main’ object, or to a ‘system origin’ empty, then rotate that.

Thanks a lot. Is there a way of getting the bounding box size for that same model also?

Select the group and read off ‘dimx/y/z’ in the ‘n’ window in object mode?