1. Log into SharePoint Site Collection as the Administrator. This will be the email@example.com credentials.
2. Select the Site Actions drop down and choose Site Settings.
3. Under 'Site Collection Administration' select Usage Summary. This will show you the amount of storage currently being used as well as the full storage amount.
Please note: You must be signed in as the Administrator to access these settings. Full Rights users will not be able to access this data.