Minimium diameter of an electromagnetic beam in terms of wavelength

[Uchida]

Is there a limit on the minimum diameter that a collimated electromagnetic beam must have (lasers or masers), in terms of its wavelength, or it is possible to create a beam with its diameter smaller than its wavelength?

I'm considering a colimated planar wave directly from the source, and not through a focusing lens.

 

[trurle]

Theoretically, you can create arbitrarily sharp EM pattern, but in practice beam spot with size less than lambda/6 will have an unacceptable level of "ghosting" (secondary maxima of intensity). Please search for "computational photolithography" for more details.