Potentiometric measurements using electrodes with different diameters of conductive graphite fiber (6 mm, 500 μm and 30 μm) with dimethyldioctadecylammonium tetraphenylborate (DDA-TPB) as the sensing material and o-nitrophenyl octyl ether (o-NPOE) as a plasticizer were performed. A study was carried out in order to determine whether the miniaturization of the electrode affects the potentiometric sensor characteristics. Potentiometric titration was used to balance the sensor membrane, while direct potentiometry was used to test the effect of electrode miniaturization on the response characteristics of the surfactant sensor. For potentiometric titrations, sodium dodecyl sulfate (NaDDS, c = 4*\( 10^{-3}\) mol/L was used as the analyte, and cetylpyridinium chloride (CPC, c = 4* \( 10^{-3}\) mol/L) as the titrant. Sensor response to NaDDS was tested in the range between 2,5*\( 10^{-8}\) mol/L and 5,0*\( 10^{-3}\) mol/L, and measurements were performed in ultra-pure water. Results of the response characteristics showed a sub-Nernst response of -52,2 ± 0,8 mV/decade of activity for a standard-sized electrode with a 6 mm graphite fiber diameter (DDA-TPB_A). A miniature electrode with a 500 μm graphite fiber diameter (DDA-TPB_B) showed a Nernst slope of -59,5 ± 2,3 mV/decade of activity, while the smallest electrode with a 30 μm graphite fiber diameter (DDA-TPB_C) showed a slope of -14,2 ± 1,4 mV/decade of activity and thus proved unsuitable for the determination of anionic surfactants at low concentrations. The lowest detection limit, 1,3* \( 10^{-7}\) mol/L, was obtained using DDA-TPB_A electrode. The study found that electrodes with graphite fibers of 6 mm and 500 μm are applicable for the determination of low concentrations of NaDDS and that there is no significant difference in the response characteristics of a miniaturized potentiometric sensor relative to standard size sensors. However, it should be considered that the miniaturization of the electrode can be carried out to a certain limit, at which the electrode still performs its purpose in a good quality as can be seen from the example of the DDA-TPB_C electrode, which, due to the extremely thin graphite fiber with a diameter of 30 μm, gave significantly worse measurement results.