Resize the persistent disk in the Google Cloud Platform Console:

1. Go to the Compute Engine page.
2. Click on “Disks” in the sidebar
3. Click the name of the disk that you want to resize.
4. At the top of the disk details page, click “Edit”.
5. In the “Size” field, enter the new size for your disk.
6. At the bottom of the disk details page, click “Save” to apply your changes to the disk.
7. Start server, sudo growpart /dev/sda 1 (if increase root disk)
8. sudo resize2fs /dev/sda1

Ref. https://stackoverflow.com/questions/22381686/how-can-size-of-the-root-disk-in-google-compute-engine-be-increased