Without a little more information as to what you have done, etc. it is a little hard to give advice. Have you put a border around the new map size? The Border would be the ENTITY you would be trimming to. Additionally, you would need to ensure that you work layer by layer (and sheet by sheet). I.E. Start with Coastlines, then do rivers, then do mountains, then forests, etc, Freezing each of the layers you are not working with.

Its because of that amount of work, I go with merely redrawing the map when I get to a zoomed view.