At first, I started with a 96-page A4 exercise book.
Then it got very messy because my handwriting was very bad. Also I tended to write in pencil and bits and pieces was just stuck into the book.
So I took this holidays to rewrite it. Now everything is in a black folder and it is very thick.
For part of the journal, I also used a blogging service.
My journal entries have been getting longer and longer. Just last night it was 3 full A4 pages. Sometimes it gets up to 6 or 7 pages for just a single entry.
However, the process diary/journal, for me, is not a matter of marks. It is a matter of writing down your inspirations and the processes you have gone through for the story to reach the state it has. It contains all the background information on the story, and if anyone actually bothered to read it, it will tell them a lot about the background of the story.