Poly(ionic liquid)s (PILs) show potential as new anhydrous polyelectrolyte-based smart electrorheological (ER) materials. Understanding the structure-property relationship on a molecular level is very important for guiding the design of PIL-based ER materials. In this paper, a family of (p-vinylbenzyl)trialkylammonium hexafluorophosphate-based PIL particles containing different length of substituent alkyl chains attached to immobile ammonium charged site is synthesized for especially understanding the size effect of side chains on ER property. To exclude the particle shape effect, the PIL particles are controlled to be monodisperse sphere-like morphology with a similar size. The ER property of PIL particles when dispersed in insulating oil is investigated and compared by temperature-modulated rheological test under external electric fields. The dielectric spectroscopy is finally performed to study the mechanism behind the size effect of side chains on the ER property of PIL particles. We demonstrate that the size of side chains on the charged site has a significant impact on the ER effect of PIL particles and the PIL particles with shorter side chains have stronger ER property but degraded temperature dependence, and this is related to the fact that the variation of side chain size alters the transport dynamic of mobile counterions and ion motion-induced interfacial polarization.