Ammonia gas sensors were fabricated using vertical ZnO nanorods (NR) on templated substrates using a new technique. The sensitivity was found to be 61% and with low response and recovery times of 6 and 13 s, respectively, for 10 ppm of NH$_3$. With an increase in growth duration, although the length and crystal sizes increased by 30% (600 to 800 nm) and 70%, i.e. 42 to 71 nm, respectively, the sensitivity dropped from 61 to 40%. With the increase in crystallinity, a decrease in strain was also found. The material showed weak quantum confinement,with a blueshift ranging from 20 to 120 meV for 5 mmol concentration, for an increase in growth duration from 3 to 7.5 h. Photoluminescence (PL) emissions showed broad convolution of multiple peaks corresponding to exciton transitions and defect-related transitions. A new model was developed to estimate NR size from the PL spectra. This model assumed a certain depth from the surface within which the recombinations occur due to redox reactionson exposure to gas. A change of radius from 15 to 23 nm was found when the PL intensity ratio changed from 0.042 to 1 for a depth of 10 nm, using this model.