Hello, AnonymousAlpha99
As I understand you want to add a Sort by File Size to your SharePoint document library. If my understanding is correct, then please the follow these steps:
Open your SharePoint document library > All Documents > Edit current view > tick the box next to File Size > OK
Now you will have an option to sort the files in your document library by size.
I hope I could answer your question. You may always refer to us if you need further assistance.
Best Regards,
Igor