We have all sizes in the house and I have to say I could easily justify a use-case for them all!
I've been using a Mini as a digital notepad this last while with GoodNotes and the Apple Pencil, but ironically I prefer a larger screen for note-taking. Its great as an e-reader, but because I'm not using it on the move, its tiny size isn't as much of a positive as I thought it would be. If you're used to a Pro screen, you'll notice the lesser quality screen on the Mini, I can guarantee it.
The 10-11" iPads are the best all round tablet sizes for most people and most tablet/consumption tasks. Stick a keyboard on one and you have a great ultra-portable tablet/computer as well. iPad Pro 11" is my favourite one. Again, if you have access to another Pro, I find I get really used to FaceID and the screen, to the point where I miss them on a regular or Air.
Its interesting to see others saying that they use their 12.9" everywhere. Once you get used to the size, it really does make for a great device in all situations - even for browsing the web, reading, watching movies etc on the sofa and bed if you don't want to have a laptop there.