Like Wonko Said.

If you have a map that is 500 x 800, but you only used 300 x 400 you have a "Lot of Unused territory there".

Now there is multiple ways to fix this, some hard, some not so hard. And without seeing the map I cannot tell you the 'best' method to resize. SO, you could create a new sheet like Wonko Said and call it MASK. Put i on the very bottom of the sheet list (this will make it the TOP sheet when printed). Then draw a SOLID BRIGHT PURPLE RECTANGLE covering the portion of the map you WANT SHOWING. This will make sense in a minute. I use Bright Purple as it is a color that is not often used in maps and makes for a quickly selectable entity. Second, on the same sheet in the same color draw a SECOND SOLID BRIGHT PURPLE RECTANGLE the covers and is LARGER than the entire map (i.e. if whole map is 500 x 800 make this purple 520 x 820). Now hide all sheet except your MASK sheet, and use the MULTIPOLY tool to select the two purple rectangles. and click done. Your will now have a large bright purple rectangle with a hole in it that will be your smaller map. Use the edit properties tool on the purple rectangle to make it white and then show all sheets. You now have your smaller map showing.

If you post up an image of your map or even the FCW file I could take a look at it and see how easy it is to 'trim down to proper size'.