You can see your total Gmail, G-Drive and Team Drive storage sizes following these steps:
1. Log in to your G-Suite environment as administrator, and select Admin >> Reports
2. Click on App Usage Activity
3. The following window is displayed.
From the drop-down menu Files Added select Total Storage Used (MB):
4. On the displayed page you can find the storage size by moving the mouse over the rightmost point on the graph. The actual utilized storage size for Gmail, G-Drive and Team Drive is then displayed