The pattern calls for 2 1/2" x 8 1/2" pieces. What is puzzling is that they don't seem to mention jelly rolls anywhere in the book that I have seen yet. I don't see why not, I've seen lots of braid patterns made from them.
There are no absolutes since there aren't other measurements that need to line up with them. They can be as long or as wide as you want them to be, in my opinion.