The conjugation length is a unique structural factor for oligomer-based π-conjugated conductors as it modulates their electronic structures. Herein, we demonstrated the conjugation length effects on conductivity by comparing a dimer and trimer of single-crystalline oligo(3,4-ethylenedioxythiophene) radical cation salts. The dimer showed a uniform-stacked columnar structure, while the trimer showed stacked columns of the π-dimerized donor and weaker intracolumnar interactions. Nevertheless, the trimer exhibited higher conductivity, suggesting a considerable decrease in the on-site Coulomb repulsion energy of the conjugation-expanded system.