Evan's suggestions are good. If you're having issues with the Outlook program, itself freezing up, it's probably not a mailbox size issue. The actual size of your mailbox (assuming you're on an exchange server) is managed by your help desk. This can be anywhere from 5MB (not likely) to (I think) 20 GB. What is likely the issue if the Outlook program itself is freezing up is a file called the OST. The OST is the file that holds a local copy of your mailbox. It is located in C:\Users\<UserName>\AppData\Local\Microsoft\Outlook (for Windows XP C:\Documents and Settings\<UserName>\Local Settings\Application Data\Microsoft\Outlook) and is called <ProfileName>.ost (usually Outlook.ost). What you'll probably see here is an OST file that is much larger than your actual mailbox (Something like a 2GB OST while your mailbox is only 150 MB). If this is the case you will want to make sure Outlook is closed, then rename the OST file and change the OST file extension to OLD. This will force Outlook to re-synchronize your entire mailbox from the server. I know this is a mouthful, and it really only works if you're using an Exchange Server. If you're using Outlook with POP3 or IMAP, it stores the email in a PST. If that's the case, a PST of version 2002 or earlier has a 2GB limit. 2003 and later has a 20GB limit, technically, but they usually get corrupt around 5 or 6 Gigabytes.
Good luck!
