Having tried the 11" and 12.9" with their Magic keyboards I'd recommend sticking to the 11" if you value using the ipad as a tablet. You can adjust to a slightly smaller keyboard with the 11" but the handling the 12.9" as a tablet is a chore. If you see yourself using your ipad setup more like a laptop for the majority of the time then go 12.9".
All said I don't really recommend the Magic keyboard considering its price. It marks easily and I can see it wearing badly within 12 months even if handled with maximum care. Some people don't worry about the price or longevity, especially tech reviewers it would seem, but for my money I want my tech to last for years.
I think you’re spot on with regards to whether one is mostly using the iPad as a tablet, verses a laptop “replacement.”
However, I bought this iPad when they were first unveiled, so I’m stuck with it. I’ll either go for the larger size on the next refresh, or buy the first Apple silicon MacBook Pro and keep this mostly as a tablet.
I did actually buy the very first 12.9” iPad Pro (with the Home Button) but it was gigantic, so I sent it back. The 10.5” was perfect, but I had two units develop the bright spot issue, so I got my money back and went without.
However, the Magic Keyboard is superb, and I haven’t used my iPad as much as this since first buying it months ago. If you have a compatible iPad, I’d strongly recommend the Magic Keyboard.