The property of DNA is strongly influenced by counterions. Packing a dense layer of DNA onto a gold nanoparticle (AuNP) generates an interesting colloidal system with many novel physical properties such as a sharp melting transition, protection of DNA against nucleases, and enhanced complementary DNA binding affinity. In this work, the effect of monovalent cation size is studied. First, for free AuNPs without DNA, larger group 1A cations are more efficient in inducing their aggregation. The same trend is observed with group 2A metals using AuNPs capped by various self-assembled monolayers. After establishing the salt range to maintain AuNP stability, the DNA adsorption kinetics is also found to be faster with the larger Cs+ compared to the smaller Li+. This is attributed to the easier dehydration of Cs+, and dehydrated Cs+ might condense on the AuNP surface to reduce the electrostatic repulsion effectively. However, after a long incubation time with a high salt concentration, Li+ allows similar to 30% more DNA packing compared to Cs+. Therefore, Li+ is more effective in reducing the charge repulsion among DNA, and Cs+ is more effective in screening the AuNP surface charge. This work suggests that physicochemical information at the bio/nanointerface can be obtained by using counterions as probes.