I went through this debate internally for a bit, but between both the 11 or 12.9, not just for myself but also for my wife.
I work in tech, finally moved off my MBP 15 2015 (great machine, but both RAM AND storage full at 1TB = too many beachballs, so it became time) to an MBP 16 32GB, and had an aging Air 2.
My wife teaches, and as many know - this is pretty much remote at the moment. I'd already nudged her back off a craptastic Dell school issue into an MBA, but she could really benefit from an Apple Pencil for some of the recordings and sessions she was doing, while her own iPad of choice had been the Mini.
For me, I can't use either IPP as my desktop replacement - no VMs, no x86 compilers, no SQL tools, etc...but I wanted to see if I could get to 'good enough' for 'travel work' - emails, Word, Diagrams, presentations, etc. and wanted to use Sidecar as a 3rd screen for myself. I waffled a bit on the 11" vs 12.9" but decided I'm going to use it daily as a sidecar screen, daily for bedtime/relaxation tablet use, and sometimes for travel with real keyboard usage. I went with the 12.9 for myself but had some hesitation on the decision. It was the right choice for me, and I did pick up the Magic Keyboard although it's insanely priced - and for me, I couldn't imagine using a keyboard much narrower - the 12.9 MKB is basically the same width as my MBP keyboard, although sans ESC key and Touch Bar or function keys.
Why the 12.9 works for me
- sidecar usage for 10+ hours a day - the bigger screen is nice, and I don't care about the weight.
- couch or bedtime consumption - no problem, although I need to find a real case that also works with the MKB for some level of protection and the MKB isn't a winner for in bed/couch reading so I wind up removing (magnetic - quickly) from the MKB
I can happily give away or sell my 10.5 Air 2 now, with zero regrets, although I do need to sort a case for more casual tablet use for my sanity/fear of dropping, etc.
For my wife, the choice was a bit harder. Her preference was always for the iPad mini from the start and she uses it a lot for reading on the couch or bed. As a teacher, she could really benefit both from an Apple pencil, and at least some sidecar use or at least at the moment, presenting from a tablet. Using a keyboard for productivity work was secondary for her, but I still had some concerns over the screen size, but the mini is a lightweight and she's more 'portable' than I am for casual use, so also didn't want needlessly large.
The 11" worked out perfectly for her. We worked out both where sidecar and standalone modes made the most sense for her teaching, and surprisingly to me, she
may be willing to give up her mini. We'll see where that winds up, but I don't think she'd be using the 12.9, let along the 12.9 with a keyboard case, for reading in bed. If she does decide she wants a keyboard, I think the 11" might be too narrow vs her MBA keyboard, but she can use my spare bluetooth keyboard as it would likely be an only occasional thing, while both sidecar, sharing of screen in meetings and pencil use, followed by casual table use, were the biggest items for her.
YMMV as always, but consider your expected actual use. My 12.9 IPP with Magic Keyboard is heavier than her MBA. Not owning an MBA but an MBP16 - works for me, but not for everyone.