I’ve played on both the iPad Pro 9.7” model and am currently using the iPad 10.5” model. I would highly reccomend you get the smart cover keyboard for whatever model you choose.
When I play War Dragons on my iPad, I don’t actually hold the iPad, instead I either play with it sitting upright on a table or desk. Or, if I am sitting on the couch, I use a cutting board as a sort of lap desk (said cutting board was purchaed explicitly for this purpose and has never been used in the kitchen). Playing with the iPad like this makes it really easy to fly hunters since you have both hands free to touch the screen, plus you can type messages so much faster, thanks to having an actual physical keyboard.
You do lose some on the outer edges of the screen, which can make killing monuments more difficult. (As you might notice in the link that @forScience mentioned up higher).
As for the main screen difference between an iPad Pro 9.7” and an iPad Pro 10.5”, the active screen area on the 10.5” goes much closer to the edges of the iPad, so even though the iPad is not physically all that much bigger, the screen feels noticably larger.
Also, Apple pencils work great when flying sorcerers in game. I notice that I can swipe faster and easier with that rather than trying to drag my finger across the screen. Sometimes I use the pencil with warriors too. Obviously fingers are best when flying hunters