I agree with the recommendation to go for 11x14.  
Nice for backgrounded cels, and no reason to risk 
your cels getting bent when you put them into the 
pockets.

There's a very nice comprehensive guide to cel 
supplies (bags, books, portfolios, etc.) and where 
to find them at http://www.oion.net/plug/qnd/
cel-supplies.htm

--Atiya