How do I check my current mailbox size?

You would use Mailbox Cleanup tools. In Outlook 2007, click Mailbox Cleanup on the Tools menu, and click the 'View Mailbox Size'. Unfortunately, the data totals are listed in kilobytes (KB), so it may seem misleading. Remember that roughly 1,000 KB is 1 MB (megabyte). You are allowed 500 MB in your mailbox, so that's equal to 500,000 KB. For example, a mailbox that is 39,500 KB in size translates to 39.5 MB.

You will see also see two tabs in the Mailbox Cleanup window: one tab is for the Local Data, one is for Server Data. Keep in mind that with Exchange, we are operating in 'cached mode', this means that all your mail account data you have is transmitted to your desktop from the servers in Exchange. It is thus possible for your Exchange mailbox to have a different amount of data than on your local machine, so it's important to check both to insure you are not at risk of reaching your quota.

Remember, all users have a 500MB (appx. 500,000 KB) mailbox quota, and this is enforced in the 'Server Data' areas, not on the 'Local Machine'.

Additionally, from the Mailbox Cleanup dialog box, you can use any one of the following features:
• Find items that are older than a specified number of days.
• Find items that are larger than a specified size.
• Move old items to the archive file by clicking AutoArchive.
• Empty the deleted items folder.
• Delete all alternative versions of items in the mailbox.