Note: You must be an Owner of the Team to view Storage.
In the Teams Desktop App, click on your Team.
Choose Files.
Choose Open in SharePoint.
This will open SharePoint Online. In SharePoint Online, choose the Gear Icon in the top right hand corner.
Then choose Site Contents.
Next choose Site Settings.
Under Site Collection Administration, choose Storage Metrics.
On the Storage Metrics page, you will see how much storage you have used and available storage in the top right hand corner. 