What's the best iPad Pro size for artists?

It really depends on your personal preference.

Size

The 12.9-inch version is more enjoyable to draw on compared to the 9.7-inch because of the larger drawing surface area. It's like drawing on an A4-sized paper vs A5.

Remember you still have to consider the user interface taking up space. The surface area of the big iPad Pro is two times larger than the smaller iPad Pro. Even after having palettes around the screen, there's still ample space to draw on. Personally for me, I find myself zooming in less to draw. With the small iPad Pro, you may need to zoom in more often.

RAM

The 12-9-inch has 4GB RAM compared to the 2GB RAM on the smaller iPad Pro.

The amount of RAM will affect certain software features. For example when working with Procreate on the bigger iPad Pro, you can get 57 layers when working on an A4 canvas while you can only get 19 layers with the smaller iPad Pro. Apple's iOS is great at managing memory but if you're a heavy user, you might find the iPad Pro slowing down as you open a lot of apps while working on a heavy digital graphics file at the same time.

True Tone

This is a new feature introduced in the smaller iPad Pro.

Apple says that the smaller iPad Pro will note the ambient lighting and adjust its screen "to match the light in your environment". It says "reading anywhere is more natural and comfortable — almost like looking at a sheet of paper".

The benefit to artists is not clear. When you pick a colour, you pick it because you want that colour. However with Tru Tone, it's going to adjust the screen's colours so basically that means you will get a colour shift in your artwork. If you're primarily drawing on a white screen, then perhaps the Tru Tone will make you feel like drawing on actual paper because of the shade, but when it comes to digital painting, there's going to be that unwanted subtle colour shift.

Portability

The smaller iPad Pro is clearly more portable. It weighs 437g vs 713g.

Let's talk about putting the iPad Pro on the lap to draw. I can't hold the big iPad Pro for a few seconds before I start to feel the strain on my hand. You definitely need something to prop up the big iPad Pro. With the small iPad Pro, it's not much of an issue but the strain will come in later.

Ideally, it's best to draw with the iPad Pro on the table.

Experience

It feels more immersive to draw on a larger canvas. It just feels more cramp on the smaller iPad Pro.

Personally, I find it more natural to draw on a larger area. And when you look at the artwork at a larger size, it's also more satisfying. Apple Pencil is almost flawless so the overall drawing experience is satisfying.

I prefer the larger iPad Pro. I believe the trade off for most people is going to be portability. As I mentioned in the beginning, which size to get will depend on your personal preference.