Outlook 2010: Reduce your mailbox size by going to File > Info > Cleanup Tools.
Two quick options to actually reduce your mailbox size are:
Find items by size and\or age and delete
Find items older than ?90? days
Find items larger than ?250? kilboytes.
You can amend and enter an appropriate figure into the given text box. Click Find.
If you want to delete all items that Outlook finds for you click one item in the results pane and use Ctrl+A (to select All).
To select non-contiguous items, hold your Control key down and then click on individual items to select your remaining choices. When finished release the Control key.
To select a contigous group, click the first choice in your list, hold down the Shift key and click the last item in the group. When finished release the Shift key.
In all cases use the Delete button on your keyboard to permanantly remove the selected items from your mailbox.
Empty the Deleted Items folder
Click the Empty button to permanently delete any tiems in your Deleted Items folder.