There have been numerous studies to demonstrate the mechanism of ionic cluster formation in the gas phase during electrospray ionization (ESI), however the mechanism of this phenomena from charged nanodroplets is controversial topic so far. It is widely accepted that small analyte ion undergo field emission from the charged nanodroplets surface, which is point of ion evaporation model (IEM) while large globular species are probably released to the gas phase via the charged residue model (CRM) where free ions are formed by vanishing of last solvent shell, droplet evaporation to dryness. However, the boundary of IEM versus CRM on the basis of size of analyte is obscure and is not widely accepted, as it has been reported that large analytes also can experience IEM. In the present work, we have investigated the formation of alkali halide (MX, M = Li, Na, K, Rb and Cs, X = Cl, Br and I) cluster cations and anions using ESI-MS. Interestingly, we have observed the polarity- and droplet size-dependent halide selectivity as (MX)_nM⁺ and (MX)_nX⁻ clusters are formed. The larger halide such as iodide (I⁻) preferentially formed the cationic clusters (MX)_nM⁺, while the smaller one such as chloride (Cl⁻) formed the anionic clusters (MX)_nX⁻. Furthermore, we have found the effect of initial size of ESI-generated charged droplet on such halide selectivity, showing that the smaller initial droplet forms the cluster ions with smaller halides. Our experimental results combined with molecular dynamics simulations propose a plausible mechanism for the ionic cluster formation. However, further confirmation of the proposed mechanism using elaborate molecular dynamics simulations on the droplet system and the complete intercomparison of experimental and theoretical results remains to be elucidated. We think this work can provide insights into the emission of ionic cluster from the the various airborne droplets, which might be helpful to understand the atmospheric molecular behaviors and cluster formations.