Step 1

In Microsoft Outlook 2010, click File.

Step 2

Under Mailbox Cleanup choose Cleanup Tools and then Mailbox Cleanup. (note: in Microsoft Outlook 2007, Mailbox Cleanup is under the Tools menu.)

Step 3

Click View Mailbox Size.

Step 4

The list that presents itself should let you see what’s holding the most data.

Alternative Way

You can also right click on a folder in your Inbox, go to properties and click on folder size.