Supposing you have big teeth.. can a dentist make them smaller?

Yes, but: "big" teeth can be made smaller, but before doing so, a thorough evaluation of the face, jaws, occlusion (bite) and teeth themselves should be performed. For example, a skeletal malocclusion with excessive show of gum tissue in the upper jaw may sometimes be misinterpreted as a big tooth problem and reducing the size of the teeth in itself may not provide the best result.