We have used an ultrahigh vacuum scanning tunneling microscope to investigate the deposition of mesoscopic iron clusters from a gas aggregation source. The size of the clusters was found to be in the range 1-7 nm. The effect of exposure of the iron clusters to the atmosphere results in a significant increase in cluster size, which we believe is consistent with total oxidation of the clusters. A specially designed quadrupole mass filter is incorporated inside the cluster source. We have investigated the sizes of the clusters deposited using different quadrupole settings and find that it is possible to size select the clusters prior to deposition. Finally we have studied the effect of sample surface reactivity and annealing on cluster distribution.