There are lots of reasons why you may want to search GMAIL by file size. I do this very often to find emails containing large attachments, so I can delete them and free up storage space.

Open the GMAIL web interface and type size: followed by the size of the attachment. In this case I am searching for emails 5MB or larger.

or you can use

This will show all emails containing an attachment larger than 10MB.

Both of the above queries can be used interchangeably.

You can even chain multiple filters to really hone in on what you are looking for

If you are interested in all of the GMAIL search operators, check out this massive Google page.