Hey AstonJ,
The 11" iPad Pro, it will feed your needs for most occasions.
In another side, the 12.9" iPad Pro when you like to split the display in a half, and for one side pick something to watch or read, and for another half put it on an app to take notes, the 12.9" iPad fits a bit better, or even writing something, and just quickly use the virtual keyboard a big and fat screen always gone help.
Also, for traveling or take it to be on your side wherever you go, the 11" it was the best. Besides, active the sidecar when you are traveling, the 12.9" seems a good second display for your MacBook.
I chose the 11" it is lighter you said. And I use it so much to taken my notes and read books and articles as well.