Thienothiophene-based molecules are the typical building blocks of high-mobility organic transistors exhibiting band-like transport behaviors. The spatial extent of the charge carrier wave functions is a fundamental quantity for characterizing such transport properties microscopically. Here, we investigate the ESR of hole carriers generated by the iodine doping of thin films of an alkylated thienothiophene derivative. The observed hyperfine-determined linewidth yields the spatial extent of wave functions over 100 molecules. The spatial extent is nearly an order of magnitude more than that in pentacene, which supports the band-like transport and is consistent with the high crystalline order of the present system.