I think that the 12.9 is big, while still being very portable. When I first got a 12.9 (2015), it felt massive. Now I use the 12.9 2018 and it doesn’t feel as big anymore even though the screen is the same size. Yes, it does have smaller bezels, but at the same time, I also got use to the size and that makes it seem smaller than it once did.
I also like that I can add a keyboard to it when I want to and it’s a full sized keyboard. I’m not trying to turn it into a laptop though, not yet, although that is a wish. I use it mainly as a handheld device in tablet mode, but for a an additional low cost, I get to add a keyboard and it becomes versatile, allowing me to type faster and use keyboard shortcuts, etc. Some people think that adding a keyboard means that people are trying to turn it into a laptop and could buy a laptop for less than an iPP and keyboard. That’s not my current aim, I still have a laptop too and I don’t intend for my iPad to be a laptop replacement just yet. One day, I hope it is, but for now, I have a keyboard that I can add for those times that I want to type. I see it as a low cost addition to the iPad that I would have anyway, to add additional functionality.
As for holding it, I usually hold it with two hands, but with the bottom of it resting on something, either a table, my lap, my stomach if I am in a recliner or bed, my knees, or even a combination such as resting on my stomach with the back of the iPad against the edge of my desk. This takes the weight off it while holding it. I don’t hold it in the air with my arms for more than minute as that is not how I hold an iPad. I can hold my mini 5 one handed, but that isn’t how I hold my 12.9 and I have no need to hold it one handed in the air. I have rheumatoid arthritis and I am not strong and if I can hold a 12.9 iPad for long periods of time, granted that it’s by resting the bottom of the iPad on something, then I think that most adults should be able to manage it.
I do think that larger touch screen devices, say a 17” iPad, would be cool. But I think that such a device for me would be a stay at home on the desk device and as I am not an artist, I don’t see myself purchasing one until I could replace my MacBook Pro with an iPad. It is getting closer and closer....but there are still things that I can’t do on my iPad, especially since I am a programmer and would like to see some programming IDEs on it, including XCode. Until then, I don’t need a larger iPad. I’m sure that some people, including artists, would like a larger option though.