I use Marvin on all 3 iPad-sizes – 8-inch, 10-inch and 13-inch – and I appreciate them all. (Not to mention the 5.5-inch iPhone 6S Plus.)
I agree that for linear fiction reading, the iPad mini seems ideal. However, especially for complex non-fiction books, where you often appreciate being able to see as much text at a single glance as possible, the 13-inch iPad Pro's screen suits me best.
Also (funnily enough), I prefer the 13-inch Marvin for reading
boring books, even if they are fiction – you see, boring books annoy me, and I typically can't wait to get rid of them by finishing them as soon as possible: and the 13-inch screen creates the comforting illusion that there are fewer pages left to go through that particular ordeal.
