Hi,
Pixels refer to the lights in our screen. World Units refer to nothing outside Unity. Their dimension in pixel depend on the camera. If the camera renders 10 World Units, and the result is a 1000 x 1000 px image, each World Unit would be 100 x 100 px. If the camera renders a 500 x 500 px image, each WU would be 50 x 50 px.
Since we also have depth in the 3D game world, it is impossible to tell how many pixels game objects will have when being rendered unless we do some fancy maths.
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