Protein-based nanoparticles offer tailored bioactivity and biodegradability that are distinct from their synthetic polymeric counterparts. Precise engineering of physical properties, especially size, of nanoparticles using electrohydrodynamic (EHD) jetting is a crucial factor that defines the fate of delivery systems in nanomedicine. Herein, we establish a systematic understanding that leads to the preparation of human serum albumin (HSA) nanoparticles with sizes as small as 50 nm. Interestingly, the addition of salt at very low concentrations, around 1–5 mm, combined with EHD process parameters, can result in narrow distributions of particle sizes that are consistently below 100 nm. At a given concentration, i.e., 2 mm, anions modulate the particle diameters that follow the Hofmeister Series as SO42− < CO32− < H2PO4− < Cl− < I−. This size reduction is primarily due to increased solution conductivity and interfacial charge density induced by salt ions during the EHD jetting process. High mobility ions compensate for the higher surface energy required to produce ultra-small nanoparticles. Tight control over the size and distribution of ultra-small nanoparticles may be critical for targeted drug delivery, as it can influence nanoparticle tropism or affect their cellular uptake.