If the folder includes subfolders, Microsoft Outlook displays each subfolder's size and the total size of all subfolders combined. If you are using a Microsoft Exchange Server e-mail account in Cached Exchange Mode, you will see folder sizes for your Offline Folder file (.ost) (Local Data tab) and your mailbox on the server (Server Data tab). Data files stored on your computer, such as Offline Folder files (.ost) aren't as efficient at data storage on the server, so the size of the data file on your computer will be larger than the reported mailbox size. Folder size is not available for Search Folders because they are virtual folders that contain views of items satisfying specific search criteria.

1. In the Navigation Pane (Navigation Pane: The column on the left side of the Outlook window that includes panes such as Shortcuts or Mail and the shortcuts or folders within each pane. Click a folder to show the items in the folder.), right-click the folder you want to determine the size for, and then click Properties on the shortcut menu.
2. Click Folder Size