Ran across an article about the upcoming sale of one of Austen's few surviving manuscripts - this one a partial MS of "The Watsons". What intrigued me for bookbinding was learning that Austen wrote on little sections of paper that she made (presumably herself, since she was very private about her writing), by cutting a sheet of paper in two, trimming the edges, stacking the two pages and folding them in half, to create 8 page sides to write on.
As her novels are quite long, I rather suspect that as she finished her sections she sewed them to one another, to keep them in order and to facilitate going back and re-reading. With only two pages to sew through, she could have done that herself quite easily, and just as easily, if she corrected so much that she needed to replace a few pages, she could cut the thread and replace the section.
The size of the pages interests me; it is said she wrote on small pages but the one photo I did see online shows that each side contains about 25 lines of about 45 - 50 characters each, and the lines are not particularly crammed together; nor is the writing. So I suspect she was making her sections by cutting down a sheet of the paper that would have been normally used at the time - perhaps a crown or a small demy - something about 15" x 20", folded and then trimmed, would have given roughly the right size; a quarto of about 7.5" x 10" (19 cm x 25.5 cm)
When the auction details are posted I will find the size and see how close I came.