Interesting that this subject has come up. The credential that I got from the pilgrim office in SJPP (a folding credential) had more than enough space to get me to Santiago. On the other hand, my cousin / walking companion, whom I met in Roncesvalles, and who got her credential there, received one in booklet form. I can't confirm this, as I didn't see it, but I have little doubt that she ran out of space before we got to Santiago and must have had to get another one somewhere along the line. Personally, and if possible, I would prefer to have all my sellos on one credential.
BTW, my understanding, after the fact, is that two sellos per day is necessary only if one starts the Camino at the 100 k. point. In any event, I often forgot to get the two sellos per day after Sarria, and it made no difference to my receiving my compostela.
I will post the exact dimensions of the folding credential later when I have time to find it, unless someone else beats me to it before then.