Abstract We extend the microscopic self-consistent cooperative hopping theory (SCCHT) of activated ion dynamics in polymerized ionic liquids (PolyILs) and glasses based on a trained polymer physics model to provide an understanding of the effect of cation size on the physical barrier and ion relaxation time as a function of temperature. Speeding up of ion hopping with increasing cation size due to reduction of the Coulomb association energy is shown to effectively win over slowing down of hopping due to a sterically induced increase of the number of anion-cation neighbors in the Coulomb cage. The subtle tendency of the physical cation barrier to saturate as ions become large enough is captured, as experimentally observed. A temperature-dependent Coulomb cage cohesive energy is shown to be the key organizing variable for different ion sizes in polymers. A two-regime super master curve behavior emerges for a range of cations and polymer chemistries. An initial application of the proposed master curve to boron anion-based lithium PolyILs is performed that supports the idea it can be useful for understanding and designing other classes of PolyILs not involving the TFSI anion. Synthetically controlling five key factors is suggested to hold the potential for achieving room-temperature superionic conductivity. New routes to realizing this seem feasible based on manipulation of the PolyIL monomer steric bulkiness, or using random copolymers of anions and neutral monomers of different volumes, to decrease the effective Coulomb cage cohesive energy via simultaneous reduction of anion-cation electrostatic association and steric frustration of the number and tightness of cohesive contacts.