It just gets dimmer, which eventually imposes a practical
limit.
=========================
To anticipate a possible follow-up question:
Loosely speaking, the Huygens construction is a sufficient
explanation.
OTOH, strictly speaking, the original Huygens construction
cannot possibly be correct. We know this because the
wave equation is a second-order differential equation,
and therefore requires /two/ boundary conditions.
Physically, this is well illustrated by the single-slit
diffraction experiment. The physics is different depending
on whether you have a slit in a /black/ barrier or a slit
in a /reflecting/ barrier ... but the Huygens prediction
is the same in either case.
This problem can be fixed using a somewhat more elaborate
construction:
David A. B. Miller
"Huygens's wave propagation principle corrected" http://www-ee.stanford.edu/~dabm/146.pdf
==================
For a slit in a /reflecting/ barrier, it is relatively easy
to get a valid solution to the wave equation.
In contrast, if you want an /absorbing/ barrier, things
get quite a bit trickier. It is impossible to have
something that is very black and very thin (in the direction
of propagation). I suspect this imposes another type of
limit on how small (in the transverse direction) you can
make your diffraction slit. http://www.av8n.com/physics/white.htm#sec-black