Checking mailbox quota
Using Desktop Interface
There are to places to see your usage.
1) Bottom left provide a percentage number and graphic representing percentage use.
2) Settings icon (look at top right of page) >> Options >> Mailbox/Disk quota is indicated here. Hover over the graphic representing percentage usage to get actual numbers.
Using Tablet Interface
Mailbox/disk quota is displayed at the top of every page
Using Mobile Interface
Mailbox/Disk quota usage is not available.