Subnanometric metal particles exhibit anomalous chemical activity, suggesting innovative applications as next-generation materials. However, a precise synthesis and detailed characterisation of these subnano-materials remain a major challenge. Here we summarise recent works on the synthesis of size-controlled tin (Sn) oxide subnanoparticles (SNPs) using the dendrimer template method, and on their detailed characterisation. Size-controlled Sn oxide SNPs (Sn₁₂, Sn₂₈ and Sn₆₀) have been synthesised and they showed a size-dependent compositional change containing not only stable Sn(IV) states but also metastable Sn(II) states so as to form subnano-scaled particle shapes. Detailed vibrational characterisation of SNPs was achieved by employing subnano-sensitive Raman spectroscopy for spectroscopic characterisation. Combined with density functional theory studies, the inherent subnano-structures of Sn oxide SNPs have been elucidated for the first time. Furthermore, the size-dependent activity of Sn oxide SNPs upon CO oxidation was rationally explained from the simulated structure of Sn oxide SNPs. A detailed understanding of the chemical and physical nature of subnano-materials facilitates the rational design of SNPs for practical applications such as catalysis, biosensors, and electronics.