Portfolios and cel books come in many sizes, it 
depends on what style you're looking for.  I 
believe I've seen paged portfolios up to 27" on 
one side (sorry, no exact numbers ^_^;; ).  I 
suggest browsing an online art store (dickblick, 
utrecht, etc) for a good idea.  With the range of 
books comes a range of prices of course.

The popular Itoya brand and style has a 14 x 17 
book.  It might be what you're looking for.

Keys