Google Apps Sync for Microsoft Outlook allows you to use Microsoft’s mail client with Google Apps. While it does have some limitations, it is quite useful for those who are not quite ready to use Gmail’s web-based interface.
You’ll then have to option to modify the size to 1GB, 2GB, 4GB or unlimited.
There is one thing to note about changing these sizes. Once your local mail file has exceeded one of these sizes, you will not be able to revert back because the file itself has already exceeded that limit. When that happens, your settings menu may look grayed out like this.
One thing that you could try to do to fix this problem is compact your Outlook local mail file, also known as a PST file. You can read more about how to do that here.
Do you use Google Apps Sync for Microsoft Outlook? Have you changed the local mail file size limit?