Well, first you need to figure out, is it TOO BIG or TOO SMALL?
If too small, by how much? Let's say you figure out the map should be 500 miles across, and you have it 250 miles across. That's a calculates out that the new scale needs to mathmatically be 2x larger than the old map. Use the SCALE button to the left of the screen and select all (make sure everything i unfrozen on the layers, and all sheets/layers are shown), then type in the command bar 2 (for the size time) and for a scale point I always choose 0,0 (doesn't really matte but all map templates start with 0,0 as lower left).
Same for Shrinking the map.