We are presenting a method for modulating the size of CdS nanoparticles by templating their formation witholigo(guanine)n oligomers where n varied from 5 to 20. The variation in template length resulted in observable changesin the size distribution of the CdS nanoparticles. Statistical analysis of AFM images showed a general trend wherebythe CdS average height decreased for longer oligoGn and increased for shorter oligoGn. Concomitantly, shorter oligoGnyielded more dispersed populations, while longer oligoGn gave less dispersed populations. This synthetic methodologycould be extended to the synthesis of other nanoparticles and even to mixed-metal nanoparticles resulting in a powerfulmethod for fine-tuning size-dependent properties.