OWA - View total number of messages in a folder (& usage)

This figure is displayed in the top left corner of the Outlook screen. The figure is the total number of items you have in the folder you have selected.

To find out how much space you are using for your entire Mailbox (Inbox, Sent and all other folders), hover over your name in the navigation pane and the amount of space you are using/have free will be displayed.