Hello, I am trying to find out whether you get more credits for the larger size paperbacks? Thank you.

Nope.   It's always one book=one credit, regardless of the size of the book.

As Robin said, one (print) book = one credit.  The only variation is audio books (cassette, CDs, MP3) = equal 2 credits.

And that goes for heavy Hardcovers too.  

One book = one credit (regardless of size or binding).  Audio recordings are 2 credits.

Folks have already answered the question.  Since most of my paperbacks now ship for $2.63, I can afford the occasional $3.12 for a larger paperback or hardcover.  There are some threads in the Bazaar where you can list it with the requirement that someone takes an ADDITIONAL book to even out the per book shipping.  I'm thinking of doing that with cookbooks that are very heavy.

I recently received a package with 2 hardcover books in it. used 2 credits and the postage was $4.12. That works out to $2.06 each for 2 fairly heavy books. seems fairly reasonable.