We present results of simulations performed as part of the development of a gamma-ray detector module comprising a non-pixellated scintillator and pixellated photodiode detector. The simulations have been carried out to determine the effect of surface treatment and dimensions of the scintillator on the ability to determine the 2-D position of interaction. A set of 32 different combinations of surface treatments have been considered for each crystal size. Scintillator dimensions considered have been 25 mm × 25 mm ×(3-6 mm). For scintillator thicknesses at the low end of this range, an average accuracy of 0.5 to 0.6 mm is achievable for many different surface treatments. At the higher end of the thickness range, 6 mm, the average accuracy reduces to around 0.7 mm, and is more dependent on the surface treatment.