Assume an electromangetic wave with the wavelength of 1 mm and a straight wavefront of 1 cm wide. So, in vacuum, does the wavefront spread out to the sides and get wider?
P.S. It would be nice if you could explain with what methods we could produce such a precise wave shape, especially on the edges/endpoints of the wavefront as the diffraction tends to prevent it.
Due to quantum (and possibly other) effects, primarily the uncertainty principle, there will always be some spread, as explained here. It is not possible, even in principle, to produce a light beam with zero divergence, unless it is infinitely large (in which case is a plane wave). In general, the greater the initial diameter of the beam, the smaller the divergence.
The picture in the article does not accurately depict such a plane wave because the plane wavefronts are actually infinitely large, unlike what the picture seems to suggest.
