The effects of alkyl group size on the structure and dynamics of poly(n-alkyl methacrylate)-based ionomers were investigated in this study. Ionomer samples were synthesized via the reversible addition-fragmentation chain transfer copolymerization of sodium styrene sulfonate and alkyl methacrylate with different alkyl groups (methyl, butyl, hexyl, lauryl, and octodecyl groups) and with varying ion contents. The samples were characterized by dielectric response spectroscopy (DRS), X-ray scattering, and linear viscoelasticity (LVE) measurements. X-ray scattering revealed ionic peaks indicative of spacing between ion aggregates. The interaggregate spacing decreased with increasing ion content when the alkyl groups were methyl, butyl, or hexyl groups. This observation is in accordance with the trend reported for sulfonated polystyrene, which has been extensively studied. In contrast, the interaggregate correlation peak merged with the backbone correlation peak when the alkyl groups were lauryl or octodecyl groups, suggesting that the steric repulsion of the long side chains partly suppressed the interchain association. Quantitative analysis of LVE showed that the sol-to-gel transition occurred at a higher number of ions per chain for samples with longer alkyl groups, lending support to the idea that long side chains generated increased steric repulsion. In addition, the plateau region broadened gradually with increasing alkyl group size, reflecting an increase in electrostatic interaction energy among the ionic groups. This increase was attributed to a reduction in the polarity of the polymer medium as the size of the nonpolar alkyl group increased, as detected by DRS.