My iPad is 13" and having a a display about the same size as a sheet of A4/letter paper is nice. It's so nice to have a full size display for marking up pdfs.
Now that I've gotten used to the iPad with Goodnotes, I'm not sure I'd get a 13" Remarkable. I'd probably buy one, but I suspect their notes app would be enough of a downgrade that I would probably return it.