- In the registry, go to HKEY_CURRENT_USER\Software\Google\Google Apps Sync\
- Click Edit > New > Key to create a new key folder.
- Specify EmailSync as the name of the key, and press Enter.
- Select the EmailSync folder you created.
- Click Edit > New > DWORD (32-bit) Value.
- Specify MaxStorePstSizeMegaBytes as the new value, and press Enter.
- Right-click the MaxStorePstSizeMegaBytes value you created and select Modify.
- In the Edit DWORD (32-bit) Value box, under Value data, specify the maximum size of the mailbox, in megabytes.
For example, 4000 MB = 4 GB. - Click OK.
https://support.google.com/a/users/answer/184126