Putting the list here for reference:
2x2: Standard enemies, Pacman, bowties
4x2: OBEY, LIES, numbers, horizontal lines that make an oval shape, smokestack (on top of clouds stage)
2x3: Trenchmen, skeletons, headless men
3x3: Yes men,
3x4: Stop signs, Ghost
4x3: Clouds
4x4: Brass men, Lies projector, speaker, large circle
6x2: TRUTH
8x2: BOOO
8x6: Large building, large monster