What size round cake pan can you substitute for a 9 by 13 inch?

A 9x13 pan has a much greater volume than a round cake pan... if you need to use round pans you would probably be better off substituting two 9' round pans for one 9x13' pan.

Ex.: The volume of a 9x13 pan is 9*13*1.5 = 175.5

Volume of a 9' round pan is 3.14*4.52*1.5 = 95

So a 9' round pan will hold about half the batter of a 9x13' pan.