Using a plasma-gas-condensation-type cluster source, we could obtain nano-scale clusters with the average diameter between 2 and 13 nm. In order to detect such large free clusters containing 1-10(5) atoms, we have constructed a time-of-flight high-mass spectrometer (TOF-HMS). The size distribution of the Nb-n clusters observed by the TOF-HMS is in good agreement with the result of the transmission electron microscope observation. The mass spectra have been measured with varying the operational parameters of the cluster source. They are shifted to the smaller size direction and the mass distribution becomes narrower with increasing the He gas flow rate, V-He, and keeping the Ar gas flow rate, V-Ar, constant. These features are attributable to the decrease in the residence times of gaseous species in the cluster source. The cluster size increases with increasing the Ar gas pressure and lowering the temperature of the cluster source.