I can tell you from experience with the 12.9", that it's a wonderfully sized screen, but when using it without the Magic Keyboard you basically always need to have to propped up somehow. Like against your chest when you're laying in bed or propping it up with your legs somehow. Holding it with one hand is basically impossible. Typing on it without a keyboard, so using the on screen keyboard, is also a huge pain. On a normal iPad it's pretty doable by holding it with two hands, or with one hand and typing with the other. But both options don't work on the 12.9". With two hands the center of the keyboard is too far away, and with one hand holding it's too heavy. So basically you have to put it down somehow to type on it.the other thing about trying in store is that it has the security tag stuck to it. The 13" really in quite insane to feel in person. Its pretty straightforward for me...if youre actually using it for work in any way or for drawing at a desk 13" all the way. for handheld or media consumption while NOT at a desk then 11"
It's also a lovely big size for handwriting or drawing. However the camera bump is a little unfortunate as it makes the iPad wobble a bit, which is not a huge deal but it's slightly annoying. If you're going to be taking a lot of notes, the folio is nicer than the Magic Keyboard because you can flip it open and get a mostly stable surface.
I've tried both the 11" and the 12.9" and when it comes to size of videos - people say the difference is small, but really it's about 30% larger with 16:9 content. That's quite significant, and you do notice it. Generally you would be holding the iPad pretty close to your face when watching content, and you really notice the size difference a lot. That's not to say the 11" isn't a nice size to watch videos on - but people claiming there's barely a noticeable difference are, in my opinion, wrong.