The formation of hydrogen poor cationic silicon clusters SinHm+ with up to ten silicon atoms in an expanding argon-hydyogen-silane plasma has been studied by mass spectrometry and Langmuir probe measurements. Sequential clustering reactions with silane, initiated by silane ions, cause their size to depend on the product of silane density and geometrical path length having possible implications for a-Si:H films deposited by remote plasmas. Reaction rates, estimated by a one-dimensional model, show no strong dependence on the number of silicon and hydrogen atoms present in the ions in contrast with rates determined by ion-cyclotron resonance mass spectrometry studies. Possible causes of the discrepandy are discussed as well as the hydrogen poverty of the clusters. The maximum contribution of the cationic clusters to the growth flux is about 6% for the conditions investigated.