As others have said, it's a personal thing when it comes to device size, as with any Apple product that comes in more than one size.
That being said, my experience has been drastically different than expected with the 12.9" Pro. As someone who has owned most of the Minis and every version of the 9.7" iPad over the years, I was expecting more of the same with a bit bigger screen.
But I was very wrong to assume that.
The user experience on the Pro is vastly different, despite looking the same on the surface. The extra screen real estate makes the interactions so much different than on a smaller iPad, and it, to me, is a far more usable and enjoyable experience. I've found that despite the much larger size, the usability is just as good as my former Air 2. I use the Pro in bed for reading and media, as well as late night design work, and during the day I use it as a primary work machine. I got the pencil and keyboard with it as well. The keyboard makes it much better for work, the pencil is great for design applications (whether sketching, drawing, or photo editing), and all together it's made it one of my favorite Apple products in years.
I highly recommend giving it a shot, even if you go in a bit skeptical like I did. I think you'll be pleasantly surprised. That being said though, shop around and get a good deal! They're out there, and just take a bit of patience to find.
EDIT: Looks like I was a little late to reply for the OP, but my response applies for anyone that might be on the fence!