This will set up Outlook to synchronize with Gmail. There is a limit on the amount of mail you can synchronize between the two systems that is set within Google Apps Sync. If your total amount of mail (including archives) is more than the default of 1 gigabyte you may wish to increase the limit. Note that it is not recommended to sync more than 4gb of data as it may slow your computer down. If you cannot sync all of your data you will still be able to look for old mail (the sync tool syncs the newest mail first) by going to the Gmail interface.
You can change that limit by: