The ability to control size and monodispersity in the 1-10 nm region is important for exploring nanoparticles in catalysis and sensing applications. A challenging issue in the thermal processing of nanoparticles is whether at this scale core size selectivity is achievable. To address this challenge, we explored whether manipulation of the stabilization energy of the alkanethiolate monolayer on nanoparticle cores could be useful for directing core size outcomes. We have found quantitative correlations between both the chain length and the concentration of the alkanethiols used in the thermal processing solution and the average size of the resulting nanoparticles. These correlations show a linear change in particle size for each methylene unit change in the added alkanethiol, and a nonlinear reciprocal relationship with changing thiol concentration. This demonstration reveals for the first time the important role played by cohesive interactions within the capping layer during thermal processing, and suggests these interactions play a significant role in regulating the thermally-activated shell desorption and re-adsorption processes responsible for mediating core growth. This understanding has significant implications toward the fine-tuning of size and monodispersity of metal nanoparticles for use in various technological applications.