changing the file type wont really affect your rendertime much, the difference is neglible. However dont render as BMPs as they suck major ass and are huge files. Render as tiffs or tgas nothing else. Jpgs etc are fine for web but not good enough for this type of work (lossy comrpession etc).

If you are using mental ray I wouldn't worry about your scene size I would be more worried about your mental ray settings. there are many settings that can cause your render to take a long time. The min and max radius settings for one. I would look into optimising your render rather than shrinking the scene size. It will be less work in the long run. Mental ray is designed to handle large scenes etc so I wouldn't think your over stretching it. Post a wire of your scene and lets take a look at it.

Alan