If you read, travel and mostly consume media, the 11" is great. I had a 12.9" and it was too big for travel and reading books. I was also using the 12.9" when they came out with the capability to use it as a 2nd monitor, but found it too buggy to be useful.
This is how I feel. For travel 11 is perfect. 12.9 would be if I was using it as a laptop replacement. The mini is great too for ultra portable travel but may be too small for many.
Also the second screen thing is way more usable after some updates on MacOS and iOS. I almost never run into an issue anymore.