How can I find out the total space that items in the Deleted Items and Sent Items folders are taking up in our Exchange Server system's Information Store?

Use the Exchange Mailbox Manager. Run the tool in report-only mode (so that it doesn't remove anything), and select only the Deleted Items and Sent Items folders. You can specify size and age restrictions if you want, but you'll get the most accurate data if you don't specify any limits. (For more information about this tool, see "The Exchange Mailbox Manager, Part 2" page 10, InstantDoc ID 47217.)