I use the 11" x 14" size for standard size cels.

I also have one of the 14" x 17" books for my 
larger oversized cels - I have a few movie cels 
that are 11x17 a few Spriggan cels that are a 
truly bizarre size.

I have my 11x14 cels in the larger book - since 
they go right to the edge in the smaller books, 
they have a tendency to flex just a little at the 
top by the binding.

--Kris