The Sum Column plugin will do what you want. But, it only does the calculation for custom columns. And the size column is a bit different to others. You would need to add a custom column for the book size with the template:
That will show the size in bytes of the largest format for each book.
Alternatively, you can create a catalog for the books in CSV format and then open that in a spreadsheet.
Or, if you are like me, just send the books to the device knowing that once it is full, they will stop. Though, it is a good idea to leave about 100MB free on Kobo devices.