Are you familiar with the Kirchhoff Integral?
If you carry out the Vectorial Kirchhoff Integral for a large circular hole (compared to wavelength) you get something like the following for the Poynting Vector
##\vec{S} \left(r , \theta, \phi \right) = \frac{1}{2} \left( \frac{E_0}{4 Z_0}\right) \left(k^4 a^4 \right)\left( \frac{1}{k^2 r^2}\right) \left[ \frac{2 J_1 \left(k a \sin \theta \right)}{ka \sin \theta}\right]^2 \left( \sin^2 \phi + \cos^2 \phi \cos^2 \theta \right) \hat{r}##
Edit: This is time averaged poynting vector hence the lack of imaginary numbers ##\langle \vec{S} \rangle = \frac{1}{2}\text{Re} \left( \vec{E} \times \vec{H}^*\right)##
As
@PeroK pointed out intensity decreases with distance.
The Kirchhoff Integral only works for large ##ka## for small ##ka## as you specified we need a different approach. One of the most important works done on this subject was done in 1944 by none other than Hans Bethe
http://www.physics.miami.edu/~curtright/Diffraction/Bethe1944.pdf
He argues (that after invoking a small argument for ##ka## and applying it to the Kirchhoff Integral) that the actual total radiation for small holes is substantially less than that predicted by the Kirchhoff Integral (after smallness parameter ##ka## is invoked). You can use the so-called "Bessel Function Multiplication Theorem to confirm that the above expression is on the order of ##k^2 a^4##.
He argues (See section comparison with Kirchhoff Integral) that the total radiation of small holes is reduced by a factor of ##k^2 a^2## where again ##ka## is small.
##(\text{Bethe}) \sim k^2 a^2 (\text{Small Kirchhoff})##
Which would make ##\left( Bethe \right) \sim k^4 a^6##, notice that this is the scattering cross section for Rayleigh Scattering (long wavelength short obstacle).
In Summary; Intensity drops off with inverse distance squared and small holes (compared to wavelength) diffract even less power than large holes (compared to wavelength)