What should be the order of size of obstacle/aperture for diffraction of light?

What is diffraction of light?

What is the order of size of a glaxy?

What is the order of magnitude of light year in etre?

If size of aperture is decreased,

Compare the phenomenon of interference and diffraction of light.

What should be the order of magnitude of the minimum frequency of electromagnetic waves that could be used to detect the presence of the planet venus.