Regularly review your Inbox, Sent and Drafts folders for large files and attachments that can be stored on an alternate drive or deleted. Attachments often occupy a majority of the disk space available in your email account.
Delete messages from your Deleted Messages folder (Right Click on the folder and choose Empty Deleted Items Folder)
Archive email you don't frequently access to your hard drive and be sure to purchase an external hard drive as a backup. For instructions on archiving email messages Click Here.
Monitor how much space you have left in your account. You will receive email notifications when you are close to running out of space in your email account. The subject of the email is Mailbox is almost full. If using Internet Explorer to access webmail, and you approach your quota of disk space, a status bar will appear in the upper left corner.
Check disk quota (mailbox size) in Outlook 2010
In Outlook 2010, to determine your disk quota usage, click the File tab and select Info in the left column. The "Mailbox Cleanup" section will display how much space you are using in your mailbox.
Check disk quota (mailbox size) in Outlook 2007
From the Tools menu, choose Mailbox Cleanup...
Click View Mailbox Size...
Check disk quota (mailbox size) in Outlook 2003, 2002, or 2000
From the View menu, select Folder List.
In the "Folder List" column, right-click Outlook Today-[Mailbox-Name], where Name is your name.
Select Properties or Properties for Mailbox.
Under the General tab, click Folder Size.
Your total usage is listed beside "Total Size (Fld+SubFld)". A detailed breakdown of each folder's size is listed below the total.
Check disk quota (mailbox size) in Outlook Web Access Exchange 2007
Log onto your email account through Internet Explorer web browser
In the navigation pane, click Mail.
Point to your name at the top of the Mail folder list.
A window appears that shows the current size of your mailbox.
This feature is only available through the Internet Explorer Web Browser. It is not supported in Firefox or Safari.