The size-tunable synthesis of metal nanoparticles (MNPs) is important for revealing their size-dependent unique properties and practical applications. Here, we developed a size-tunable synthesis method based on the LaMer model for the synthesis of a silver nanoparticle (Ag NP) in an alkylamine mixture. With this method, the diameter of Ag NPs was controlled within the 7−16 nm range via the stepwise thermal decomposition of silver oxalate (Ag 2 (ox)). The key to the success of this novel synthesis method was the removal of the counteranion, the oxalate ion, from the reaction medium by converting it into carbon dioxide gas. Through this conversion, the thermal decomposition of Ag 2 (ox) was managed and the size of the Ag NPs was controlled. Finally, Ag NP films with different particle diameters were fabricated through the Langmuir−Blodgett (LB) method to tune the plasmon resonance band generated by interparticle coupling, yielding multicolored films with various intercoupled plasmon bands.