Most normal cels are about 9" x 10.5".  These 
will barely fit in the 9" x 12" Itoya.  However, 
if there's a cel bag around the cel, you can just 
forget about it.  It adds just enough width to 
prevent it from entering the cel book.

At the local art store, the price difference 
between the two books is about $3.  $6.94 for the 
9" x 12" Itoya and $9.68 for the 11" x 14" 
Itoya.  So, you should spend the extra $3 and go 
for the bigger book.

The 11" x 14" Itoya should also accomodate normal 
backgrounded cels, which are usually 10" x 14".  
However, there are quite a few that are 10" x 
15", which would leave an inch jutting from the 
book.

Of course, there are tons of non-standard cel 
sizes.  For that purpose, I bought a 14" x 17" 
Itoya ($17.63).  This should work well for many 
unusual cel sizes.

The art store's web link to the Itoya section is 
here (the on-line and in-store prices are the 
same):

http://www.utrechtart.com/dsp_showProduct.cfm?
group=w51547

For cels larger than 17", you need to go with a 
different company.  There is nothing wrong with 
other companies than Itoya, it's just that 
they're more expensive for comparative products.  
For my super oversized cels, I got a 18" x 24" 
presentation case from Prat that included 10 
clear archival storage pages.  $82.79 at the link 
below:

http://www.utrechtart.com/dsp_display_prod.cfm?
p=w24615&g=w24611

This is probably more of a comprehensive 
response, but oh well.  One for the new archive, 
I suppose.  Later.