When you use G Suite Sync for Microsoft® Outlook® (GSSMO), your mail is stored both remotely in the cloud, and locally in a mailbox (PST) file on your computer. The cloud can hold up to 30 GB of mail (or more), which is a lot more than you'd typically want to keep on your computer. The amount of mail stored locally is instead limited by Microsoft Outlook's mailbox size.

If you run out of space locally, older mail is removed from your computer and replaced with newer messages. But all messages remain available in your Gmail account on the web. Most people do fine with a local limit of 1 GB. But to keep more mail around locally, you can choose a larger mailbox size.

  1. Click the GSSMO icon in your Windows system tray.
  2. Choose Set mailbox size limit. (This command isn't available if your mailbox limit is set using a Windows registry key—see below for details.)
  3. Select a mailbox size from the dialog that opens. Note that depending on your computer, a larger size might degrade performance when searching and indexing.

    Why is a size option not available? If your local PST file already exceeds one of these sizes, the option is no longer available in the Set Mailbox Size dialog—that is, once you select a larger size and your PST grows to that size, you can't go back. If this happens, you can try using Outlook to compact the local PST file so it's smaller, in which case a smaller size limit might become available again.

    Does the Unlimited size option really mean unlimited? No. By default, Outlook limits the size of a local PST file to 20 GB (for Outlook 2007) or 50 GB (for Outlook 2010, 2013, or 2016). So, if you choose the Unlimited mailbox size in GSSMO, your PST won't exceed 20 GB (for Outlook 2007) or 50 GB (for Outlook 2010, 2013, or 2016). To increase the limit of your PST file, consult your Microsoft documentation. (Note that if you choose an Unlimited mailbox size in GSSMO and your PST grows larger than your default Outlook limit, you might encounter sync errors. In addition, large PST files can create performance problems.)

Why is my PST file larger than the size limit?

It's possible that your PST file may exceed the size limit. This depends on your Outlook set up. Consult your Microsoft documentation to see how to configure the size limit for PST files in Outlook. You can also compact PST files in Outlook, which will help reduce the file's size. 

Can I limit mailbox size using the Windows registry key?

Yes, you or your administrator can set the mailbox size limit using a Windows registry key. For details, see Configure options via the registry. Note that you can use either the registry key or the Set mailbox size limit command, but not both.

Source: https://support.google.com/a/users/answer/184126?hl=en&ref_topic=29321#