We investigated the effect of particle size during oxidation and the sono-Fenton process in an open-air system (without air supply) containing microscale zero-valent iron (MZVI) and nanoscale zero-valent iron (nZVI) on the degradation of 1,4-dioxane (1,4-D) at neutral pH. Time-dependent concentrations of 1,4-D, H2O2, and ionic Fe were measured as a function of MZVI and nZVI loading during oxidation and sono-Fenton (without addition of H2O2). The optimal loading of MZVI and nZVI for 1,4-D degradation were determined to be 0.5 g/L during both oxidation and sono-Fenton, and 0.2 and 0.1 g/L during both oxidation and sono-Fenton, respectively. Overall, the optimal loading of MZVI in both the oxidation and sono-Fenton system showed better 1,4-D degradation efficiency compared with the optimal loading of nZVI for 6 h: approximately 60.00 and 76.65% removal efficiencies for MZVI oxidation and sono-Fenton, respectively, while approximately 47.13 and 60.98% removal efficiencies for nZVI oxidation and sono-Fenton, respectively. Consequently, the smaller size of ZVI did not enhance 1,4-D degradation efficiency during the oxidation and sono-Fenton at neutral pH.