Several solutions (and several problems):
- There is a very simple image effect script by @Miguelito that pixelates the camera image, HOWEVER, it doesn't make the format square. To make it square you either have to enforce a special resolution through Screen.SetResolution, HOWEVER, not all platforms (GPUs I think) support non-standard resolutions in full screen and will just crash when you use the method (windowed is fine). Or, you can create some letterboxing squares that match up with 64 pixel width
- Using a camera that writes to a 64x64 resolution render texture, then that outputs to the main camera. HOWEVER, in that case, unity assumes the camera is 64 pixels tall and will scale image effects or shadow mapping for example
What I will do: I'll go with 1. with letterboxing, and if windowed then I'll force the square resolution. I'll use it primarily cause I intend to use real time shadows in 3d and I need them to be sharp and crisp