This isn't really important, I did it just to see if it would work.
I made a fractal stencil that was 12000 * 12000 pixels in size. When I tried to put it on my canvas I saw nothing except the pin.
The next time I tried it it took a while for the pin to show up. So I pressed the (empty) stencil icon a couple of times out of impatience. Two more pins showed up and then Artrage crashed.
I'm gonna remove the new stencil from the stencil folder, just having the stencil be there made the program lag when I opened the Stencil browser.
The PNG image was between 8 and 9 megabytes big.
Is there a max size for Stencil resolution, and what is it?