I thought I'd answer this in detail. The credential I got from the St. Jean Pied de Port pilgrim's office in 2015 is two sided heavy stock approximately 70 cm long and 17.5 cm high (or about 28 inches by 7 inches). It is folded to form 7 "pages" per side. Page 1 of side 1 is filled with your pilgrim information; page 2 is filled with notes from the Association Les Amis du Chemin de St Jacques; page 7 has a page of the
Camino Frances. Page 1 of side 2 has more notes from the Association and page 7 has a scallop shell design that serves as the cover of the credential when it is folded. When folded it completely fits inside the
Brierley guide. So side 1 has 4 pages for sellos and side 2 has 5 pages. Each page can hold 8 typical sized sellos (but some are bigger than others and somtimes they are stamped in such a way that 8 won't fit.) The Association's credential has a header on each page for stamps but is otherwise blank.
See the pictures attached to earlier posts in this thread.
[I edited this page to include the following text of another post that can be seen farther down.]
If you want to display your stamps this is what I suggest; pick up maybe 3 of the credentials from the Association Les Amis du Chemin de St Jacques. Open each so the shell is on the right. On the left side fold the page so you can see the map and the shell both. Staple the left side. Now you have a 2 sided credential with 4 blank pages per side. For the nightly stamps just use the side with the map and shell. Use the side with your name and information for less important stamps. Each side should hold 24 to 32 stamps on the 4 pages.
The advantage of this is when you get home and open up a credential to pin to your wall it will have a decoration on each of the end pages (the map and the shell). Also it will be a bit shorter (60 cm or 23.5 inches) and may fit the available space or picture frame better. And if you still need space you can fold under either end page (or both) and still show all your important stamps. If you have two credentials to display you could have one showing the the map and one showing the shell. In that case they could be shown in a block 35 cm by 50 cm (or about 14 inches by 20 inches).
In the U.S. one standard picture frame size is 16 inches by 20 inches so that would hold two credentials with space at the top to create a title.