There is a legitimate case for expansion. The appellate districts expanded from 9 to 13, and at one time there was one SCOTUS justice for each district. So, 13 is probably justifiable.
I think legislating the size is a good idea. My guess is the GOP will try it based on my argument next time they have a chance. They would love a 10-3 majority. So, there is some urgency to do something.