The size of the snow particles involved in a powder-snow avalanche is a key parameter of the local dynamic of the flow. An experimental device has been realized to collect snow particles within powder-snow avalanches. Snow particles have been captured in the powder-snow part of an avalanche triggered artificially on the experimental test site of the vallée de la Sionne. The collected particles have been photographed and the pictures digitized. An image analysis tool to evaluate the size of the collected particles have been developed for the purpose of this study. The obtained order of magnitude is 0.2mm.