See an orthodontist. An orthodontist can tell you if your jaws relate well to each other, and if not what is the best treatment modality to harmonize jaw size.
Quite a bit. A competent orthodontist can make significant changes with orthodontics alone. See an orthodontist and have a complete diagnosis and treatment plan developed.
Jaw. It depends on your profile, tmj, bite, breathing, sleep apnea disorder. When all considered, then an orthodontist and/or oral surgeon can make a plan.
Depends. A small lower jaw may indeed be just that, small. It may also project at a very vertical angle to the skull and result in much less horizontal projection: perceived lower jaw. Similarly, a large upper jaw may make the lower appear smaller in relation. Orthodontics can change relationships of the teeth (and sometimes reduce upper jaw size), also an advancement genioplasty can bring the chin forward.