Outlook 2010 offers a simple way to check the size of each folder contained into your account. Data File Properties provides a list of information about the account in question. You can upgrade to color categories, protect account with password, and view each folder’s size.
To view the size of folders contained in an account, in main navigation bar, right-click account and select Data File Properties.
This will open Data Files properties dialog, now click Folder Size to bring up it’s dialog, which shows size of each folder size contained in selected account.