Nanosized silicon powders were prepared by gas-phase cluster agglomeration reactions in a low-pressure silane plasma. The formation and agglomeration of clusters leading to the growth of primary particles of powder were studied by in-situ techniques including mass spectroscopy and laser light-scattering experiments. These powders, generally amorphous and crystallized in a reducing atmosphere, were studied in detail by Raman spectroscopy and high-resolution electron microscopy, which revealed a very rough surface of as-prepared single powder particles with structures of 1 to 2 nm. Upon 1-h annealing at temperatures between 300 and 600 degrees C, circular contrast features, 1.5 to 2.5 nm in size, are observed in the amorphous particles, which show medium-range order. A distinct onset of crystallization is observed at 700 degrees C with structures ranging from very small crystalline ordered regions of 2.5-3.5 nm in size to fast-grown multiple-twinned crystallites. The crystallization behavior is influenced by the clusters that form primary particles. Observed sintering behavior cannot be explained by a classical approach; hence, theoretical models need to be adapted to nanosized powders.