How do I make an effect scale consistently?

I originally used low fullscreen quality and pixel rounding, but this didn't do the trick for me, since I still wanted smooth elements in the game.

imgur.com/3RVJu5O

Here you can see that the pixellate effect (on the red scarf) isn't consistent with the game window.

I know of the Platforminfo plugin that can give me the size of the game window, but I can't come up with a formula to feed to the pixellate effect parameter.