  We present new spatially resolved observations of the dust thermal emission
at 7 mm from the Fomalhaut debris disk obtained with the Australia Telescope
Compact Array. These observations provide the longest wavelength detection of
the Fomalhaut debris disk to date. We combined the new data to literature
sub-mm data to investigate the spectral index of the dust thermal emission in
the sub-millimeter and constrained the $q$-slope of the power-law grain size
distribution. We derived a value for $q = 3.48 \pm 0.14$ for grains with sizes
around 1 mm. This is consistent with the classical prediction for a collisional
cascade at the steady-state. The same value cannot be explained by more recent
collisional models of planetesimals in which either the velocity distribution
of the large bodies or their tensile strength is a strong function of the body
size.
