Very common scenario requested is to show how many documents are there across document libraries in a SharePoint site and the file sizes. Unfortunately, SharePoint does not offer this out-of-the-box even for a single document library. Sync files to desktop is one option, but all users who need to see this willl have to sync.
Luckily, the new Power BI does offer this capability, albeit not well known. I literally stumbled upon this. I have posted an article here on how-to (Assuming basic knowledge of Power BI) -
Hope this helps.