Pixelation Shader
This is the correct way (one of many) of showing pixelation as a post-processing effect.
This effect will work in any aspect ratio without any pixel size scaling issues as well as it is very minimal in terms of coding it up.
pixelate image effect in action
In order to get this to work 2 components have to be set up:
1) The pixelation image effect
2) The script - which will be attached to the camera