They measure 2.5 by 3.5 inches, and I fill them with recycled paper, sometimes blank, sometimes lined.
Each book has from 40 to 45 sheets and 80-90 pages.
I've had orders for these one at a time for stocking stuffers, for personal use, or for thinking-of-you gifts. And I have had people order bulk orders for party favors.
For more mini-notebooks: