In photoshop, create a 100x100 ducument. Then:

view>pixel aspect ratio> custom pixel aspect ratio> enter 0.5
And then
view>pixel aspect ratio correction

This should allow you to draw on the right kind of canvas.

Then before you export it you scale the height to 200% or something like that, or maybe photoshop already does that for you, I don't know...

I use Photoshop. Is this just making every pixel twice as long? 

@DioShiba Yes! And what Hapiel said. You can change the aspect ratio in programs like Aseprite or Grafx2, and since the aspect ratio on PJ is 1:1, we require a 100x200 export rather than 100x100, so that the tall pixels can be seen!

Correct!  Here's a verion as exported from Aseprite...

100x100 canvas created using "Pixel Aspect Ratio: Double-High Pixels (1:2)"

and then later exported using "Resize: 100%" (aka no resize) and "[x] Apply Pixel Ratio"

Depending on the software you're using it might be called 100x200 or 100x100. For example in aseprite you work on a 100x100 canvas with 1x2 pixels, and then once you finish you have to scale it to 100x200 before exporting.

The image you submit here has to be 100x200. As long as you do that, you should be good!

So essentially we're working with a 100X200 canvas because the pixel size we are working with are 1X2?

Tall pixels are 1x2 pixels, rather than 1x1, if that makes more sense!