Long time 11" user, and just bought and returned a 12.9" after a week or so. It was pretty confusing trying to figure out if I wanted to keep it or not.
Firstly, the 12.9 screen and size were both breathtaking. I really want the 11" to get mini LED. But the novelty of having such a big and beautiful screen on a tablet does wear off. And once the novelty wore off, I started to run into issues.
The aspect ratio and size is cumbersome. I like having a bigger screen, but it being such a tall 4:3 aspect ratio means there is a lot of size and weight that's going into portions of the screen that are somewhat unnecessary. The bigger black bars mean that while the 12.9" still displays video content meaningfully bigger, it's not enough of an upgrade to justify the extra size and weight.
I love the magic keyboard on the 11", but on the 12.9" I found it a bit annoying. The key travel is a bit too deep, and the typing is actually quite loud compared to the 11" magic keyboard and the 14" MacBook Pro keyboard. I normally don't complain about these sorts of things, but I couldn't stop noticing it.
I also found that the weight of the 12.9" magic keyboard just put the 12.9" too far in the size and weight vs capability and niceness ratio. With a smart folio, I kinda understand it, but once the thing gets to 3lbs with the keyboard, you're entering a size and weight (and price) class that just commands more capability. 11" with MK, it's like, I can't believe I can do so much on this tiny little device. 12.9" with MK, it's like, I feel like I should be able to do more with a device this large and expensive.
Finally, I found it just slightly too big and heavy to use comfortably in bed, and I'm tall with large hands. When I finally went back to my 11" I was a bit relieved, and enjoyed being able to use my iPad in bed with ease, rather than with slight discomfort and struggle.
I use two main devices now: the 11" iPad Pro (sometimes in smart folio, sometimes with magic keyboard) when I want to use it in bed, around the house, or when travelling light, or doing something/going somewhere where a small device makes sense but a laptop doesn't. And a 14" MacBook Pro when I want a bigger screen and need to do more demanding stuff. Both these devices cover all my needs, whereas the 12.9" didn't really fit in to either of those needs.
My ideal iPad would be an 11" scaled up to about 12", possibly by reducing the bezels a bit. Something that was just slightly bigger than the 11", with mini LED/OLED, and the same aspect ratio of the 11" (less black bars and height) would be perfect. Chuck the camera in the correct location, enable the smart connector to act as USB-C/thunderbolt passthrough so you could use either the magic keyboard port on the left side, or iPad port on the right side. That would be a pretty meaningful upgrade, and given there are rumours that the 12.9" could be replaced by a 14" iPad, it stands to reason there is at least a small chance the 11" will be sized up to 12".