You have to periodically empty your Deleted Items folder. Once items are Deleted from your Deleted Items folder they are still user-recoverable for two weeks. You don't have an Option for the Sent Items folder like you do for the Deleted Items to empty on exit. You're just going to have to delete items from Sent Items periodically. You can sort on any column in the listing by clicking on the column header.
Attachments are a big culprit in space usage.
You can even click on the paperclip icon to sort all of your messages with attachments to the top of the list. Click on the first message, and then Ctrl-Click on each individual message you'd like to select before clicking Delete. Sent Items off-line First, follow the instructions to setup Personal Folders separate from your server mailbox. Second, create a folder in your Personal Folders for storing your Sent Items. Right-click on Personal Folders and select New Folder.