theres no way to remove the "wasted space," it resizes itself to the proportion of ur map.
Are you happy with the size of your map?
Then don't touch it. There's nothing you can do to remove it.
Are you no happy with the size of your map?
1) Do what As de Espada says.
2) Highlight your whole map, then go to properties tab, then scale down the size of your map.