Ionized Cluster Beam (ICB) deposition has received considerable attention in recent years because of its potential application to low temperature semiconductor and metal film growth. To better understand this deposition process, however, it is necessary to determine the properties of the cluster source and the cluster size distribution obtained. This paper discusses the design of a time-of-flight mass spectrometer with single atom resolution for the measurement of cluster size in a conventional ICB deposition system. Results have been obtained for silver and germanium deposition. In both cases most of the clusters have been found to consist of less than 35 atoms. With the high resolution spectrometer it has been possible to show that certain cluster sizes are more stable than others.