We investigated the effects of Si nanowire (SiNW) dimensions and their surface modifications on the pH-dependent electronic transport characteristics of SiNW Electrolyte-insulator-Semiconductor Field-Effect Transistors (EISFETs). The threshold voltages, Vth's, of all devices were extracted from the Id-Vg characteristics with Vg applied to the reference electrode immersed in different pH solutions, and their pH-dependences were analyzed for various devices. We found that our devices produce the systematic pH-dependence of Vth with respect to the SiNW's length and show significant changes in a linear pH region and a pH sensitivity upon the Si surface modifications. Particularly in the case of the APTES-treated surface, the linear variation was observed in the wide region of pH = 2 to approximately 11 with the sensitivity of 54.7 +/- 0.6 mV/pH. Also we compared our data to a theoretical result based on the Gouy-Chapmam-Stern-Graham model and found a reasonable agreement between them.