[GSlob]

Well, small lasers [easier to collimate] in rangefinders have beam divergence in the range of 0.5 millirad [aka MIL from mil-dot scope]. Assume the same here and you will not be far off. Thus at 5 km distance the beam would be something like 3 m in diameter. If at the source it is about 1 meter wide, then energy density at 5 km is 1/9th of that on the laser mirror.

[Paul Ross]

Since you are conversant with some of the issues, it first needs to be pointed out that the THEL has already proven as an capable missile, warhead, and artillery shell-defense. So the technical objections have already been worked through, albeit they pose issues of limits on range. The theoretical basis for managing beam divergence is discussed in here at note 48:

Richard Saunders, et al., Lasers Operation, Equipment, Application, and Design (New York, NY. McGraw-Hill Book Company, 1980), p. 10. For reference, The lower limit on the divergence (or spreading) of a laser beam is a function of the wavelength of the light and the size of the aperture. And divergence can be reduced by either shifting to shorter wavelengths (higher frequencies) or increasing the size of the aperture. Lasers whose divergence is reduced to the theoretical minimum are categorized as diffraction limited.