The outlook folders (.pst file) should be kept under 2 gigabytes.
If Outlook will not start or freezes during startup then the following procedure might help to recover email functions:
1) Start Outlook in safe mode by holding down the ctrl key while starting Outlook.
2) Remove unwanted email(s) from the inbox.
3) Close and restart Outlook in normal mode.
If that does not solve the problem then start Outlook in safe mode and clean up Outlook folders using Tools/Mailbox Cleanup or by deleting old emails which are no longer needed from the Outlook folders.