How to sort gmail by attachment size??

Well, there's NO way to do it. Full Stop. Unless you wan to import your emails to your computer email client like Outlook.

Otherwise, if you are like me, who just want to delete some emails with attachments that you do not need anymore, you can search your gmail with the key word “has:attachment”. Then, do a manual scan down the list for large but useless attachment. This is the most direct way I believe.