  We present a method to analyze images of the coma of
67P/Churyumov-Gerasimenko obtained using OSIRIS, the main imaging system
onboard \textit{Rosetta}, where dust aggregates can be seen as bright tracks
because of their relative velocity with respect to the spacecraft. We applied
this method to 105 images taken in 2015 July, 2015 December and 2016 January,
identifying more than 20000 individual objects. We performed a photometric
analysis of them, finding their phase function. This phase function follows the
same trend as the one found for the nucleus, consistent with the detected
particles having a size larger than $\sim 1$ mm. Additionally, the phase
function becomes shallower for increasing heliocentric distances, indicating a
decrease in the mean agglomerate size. In order to characterize the
agglomerates observed in the image, we developed a simplified model for their
ejection and dynamics in the coma, and generated synthetic images based on it.
We solved the inverse problem by finding the simulation parameters that give
the best fit between synthetic and real images. In doing so, we were able to
obtain a mean agglomerate size $\sim$ dm and initial speed $\simeq$ 1 m
s$^{-1}$. Both show a decrease with increasing heliocentric distance, sign of
the reduction in activity. Also, the sizes obtained by the comparison are not
compatible with ejection caused by water activity, so other sources have to be
invoked, mainly CO$_2$.
