I had (still have) an iPad Air 2 before getting the new 12.9 Pro. I bought the Air 2 exclusively for painting with Procreate, as I actually prefer the Mini for general use. It served me well as a portable digital painting studio, but since getting the 12.9 I haven't used it at all. I was planning on keeping the Air 2 as my portable device, but I'm finding that I prefer to carry around the 12.9 to enjoy the considerable benefits of painting on a larger screen. Yes, it's less comfortable to hold one-handed, but for me the trade-off is worth it. Using the 12.9 has required some adaptations -I have to use a rucksack rather than my small shoulder bag, I use a special stand for watching videos in bed, and when painting in landscape mode I have to sit down with the iPad on my knees rather than stand and hold it in one hand - but for me, all this is worth it. But for all its 'pro' features that the Air 2 lacks (promotion, Truetone etc.), the only features that have really made the upgrade worth it for me are the larger screen and, to a lesser extent, pencil support and the ASK. So I think that if I'd replaced the Air 2 with a 10.5, it would have felt like a slightly less significant upgrade(contrary to the OP, my Air 2 still works just fine). I know that most people feel differently about that, so that's just my personal standpoint. To me, the 12.9 is not just an upgrade, but actually feels like a totally different type of machine -so I can completely understand why some people are getting both.
To summarise, I think that the 12.9 is hands down the better device for drawing/painting (it's not that you can't get similar results on a smaller tablet, it's just that it's a much more pleasurable experience on the larger screen, despite the extra effort it requires when out and about), but for general consumption I would personally recommend the smaller size, especially if you intend to use the device for travelling.