The Stark tuning rate, STR, of free and chemisorbed CO and OH-on Pt(111) is theoretically studied by means of a cluster model density functional theory, DFT, approach with the hybrid B3LYP potential for the exchange-correlation functional. The STR is obtained by explicit calculation of the corresponding vibrational frequencies and of their variation with respect to the intensity of a uniform external electric field. A point of special concern is the influence of the size of the model employed which has been tested by progressively increasing the number of metal atoms employed in the surface model. For both CO and OH- on Pt(111) results show that, except for extremely small cluster models, STR values do not appreciably vary with respect to cluster size. Therefore, the use of a cluster model does not introduce artefacts that may mislead the physical description of this property.