Another really popular question I see is “How much space is my SharePoint site using?”. Here’s how you find out for a complete Site Collection (unfortunately, you can’t easily do just do single sub-site, you need to do the whole Site Collection).
Navigate to the top site in the Site Collection in question as an administrator.
Select the cog in the top right of the window just to the right of your name.
From the menu that appears select Site Settings.
In the Site Settings page locate the Storage Metrics option.
You’ll find it at the bottom right under the Site Collection Administration section.
That will take you to a summary page as shown above. In there you can see this size of things like Document Libraries, Lists, etc but you can also see the size of sub sites.
The listed items are hyperlinked so if I click on the Document Library ‘Documents’ you’ll be shown the storage report for that element as shown above (i.e. all the documents stored inside it).
If you select a sub site you will be shown a storage report for this area.
Thus, it is easy to look at the storage usage for a Site Collection and then start drilling down into specific areas.
You can do this for any SharePoint Online Site Collection you have INCLUDING OneDrive for Business.