Selling an original ebook

Hi, I'm a photographer and I'm running a photography blog. I want to try selling small e-books as articles in some of the publications for about $ 5. I've seen other bloggers doing it.
The question I have is about the size of the book. I'm not looking for something serious. It should be more like a manual with examples, tips and tricks, well presented, etc. What size should I take into account?