That is the upper limit to the canvas, but very few devices would even be able to work with that comfortably, so we didn't really expect people to run into it very often! (Honestly, I'd forgotten there was a limit at all because nobody ever hits it). The fact it won't let you go over 32,000 might be something we can change in AR6/future updates, but I don't think you'll be able to draw very easily anyway above that.

ArtRage 4.5 capped out at about 18,000 pixels, for comparison. ArtRage uses a lot more memory than Photoshop because it's doing a bunch of different things in the background, so it will never let you work as large as you can in Photoshop anyway. If you have trouble working at a very large size, you might want to try recording a script at a smaller size and playing it back, even if just for the 'fill in the background' stages. Other than that, you'll have to increase the size elsewhere.

Also as markw says, you don't usually a need a very high DPI if you are printing out a very largescale canvas; people are going to be standing a foot or more back instead of viewing it up close the way they would with a greeting card or something.