One of my previous article explains as to how to find the mailbox quota information from Outlook 2010. Another easier way to find the information is to turn on the "Quota Information" in Outlook 2010.
The quota information shows the amount of free space that is left in your mailbox quota and the number of items you have in each folder, say Inbox, Sent Items etc as you navigate from one folder to the other.
The feature is turned off by default. In order to switch it on, right click anywhere on the bottom Outlook 2010 bar.
Click on the first option "Quota Information" and you will see that it switches to the "On" mode.
You can now see the amount of free space left and the number of items you have in each folder, in the bottom left hand corner.
Useful feature and right in front of you!