Hello!
Here is a video by project-Shasta, which should help you out:
The only thing he does is putting in into a material function, which you can create by right clicking in your project’s folder view and select “Materials & Textures” -> “Material function”.
For you convenience I made the material without a material function and it looks like this:
https://i.imgur.com/u5QHSrH.png?1
The “PixelSize” is just a parameter. You can create one by right clicking in the material editor and type in “scalar parameter”.Give it a name and a number. If you don’t want to recompile everytime in the material editor and you only want to change the pixelation, just create a material instance and apply it to your object.^^
Hope it helps 