Just to add for future Googlers, the custom view didn't work for me as it only sums up files in the folder, and doesn't do any recursive sums. But Tim's comment on the OP's question was helpful for me, and may be helpful for others who end up here.
The Site Settings > Storage Metrics section shows a recursive total size for folders and libraries. Apparently Microsoft removed the navigation path from Site Settings, as all paths on Googled answers don't exist anymore. But using Tim's hint above I found the full URL to be:
Similarly, you can get there from Office Admin Center:
- Go to O365 Admin Center
- On left sidebar navigate to SharePoint Admin Center
- In SharePoint Admin Center left nav, go to Sites > Active Sites
The direct URL for this one is: