Question

what must be the size of the obstacles to get diffraction effect

Answer 1

Diffraction occurs only when the size of the obstacle/aperture is comparable to the wavelength of the light used. Greater the wavelength, more pronounced is thediffraction effect. It is due to this reason that diffraction effect is very commonly observed in sound.

Answer 2

Diffraction occurs only when the size of obstacles is comparable to the wavelength of light used