Login as the Email Administrator account for your domain

Go to 'Reports' on the left side navigation menu

Go to 'Domain Summary Reports'

Go to 'Traffic Reports'

Click on ‘Disk Usage’

Total Disk - This is the amount of storage space your user/email account is currently using

Max Disk - Total amount of storage space allocated and available for your user/email account

This will show you how much email storage space is allocated to each of user/email account and how much of that space is currently being used.