Size-controlled Cu clusters are formed in a system which combines pulsed magnetron sputtering and gas condensation at room temperature. The discharge repetition frequency (0.1–25 kHz) and the duty cycles (20–90%) of the magnetron sputtering are varied systematically, the influence of discharge current (100–800 mA) and the pressure in the condensation tube (25–90 Pa) is also investigated. The cluster mass is determined by a quadrupole mass filter in the aggregation tube, and the cluster size by atomic force microscopy (AFM) imaging of deposited clusters. For all preparation conditions, the cluster mass shows a log-normal distribution. A non-monotonic frequency dependence with a maximum at 1 kHz and 20% of duty cycle is observed (about 105 amu, or cluster diameter 8–10 nm). By adjusting discharge frequency and duty cycle, the cluster mass can be decreased by one order of magnitude. We suggest that this effect is caused by energy dissipated into the aggregation tube; and find a critical buffer gas temperature Tg-cr which limits cluster growth. This view is supported by the constant cluster mass flux which does not change on variation of discharge repetition frequency or duty cycle. This feature indicates mass conservation.