This is a short tutorial on how to view the mailbox size in Microsoft Outlook 2010. It breaks it down by folders.
Outlook Mailbox Size
In Microsoft Outlook 2010, click File.
Under Mailbox Cleanup choose Cleanup Tools and then Mailbox Cleanup. (note: in Microsoft Outlook 2007, Mailbox Cleanup is under the Tools menu.)
Click View Mailbox Size.
The list that presents itself should let you see what’s holding the most data.
You can also right click on a folder in your Inbox, go to properties and click on folder size.
You can also read Microsoft’s article “Manage your mailbox I: Find its size and trim it down.”