In order to design and evaluate the behaviour of a numerically optimised wave energy converter (WEC), a recommended procedure is to initially study small scale models in controlled laboratory conditions and then progress further up until the full-scale is reached. At any point, an important step is the correct selection of the wave theory to model the dynamical behaviour of the WEC. Most authors recommend the selection of a wave theory based on dimensional parameters, which usually does not consider the model scale. In this work, the scale effects for a point absorber are studied based on numerical simulations for three different regular waves conditions. Furthermore, three different wave theories are used to simulate two scales 1:1 and 1:50. The WEC-wave interaction is modelled by using a numerical wave tank implemented in ANSYS-Fluent with a floating object representing the WEC. Results show that the normalised difference between 1:1 and 1:50 models, keeping the same wave theory fluctuate between 30% and 58% of the WEC heave motion and that a wrong selection of the wave theory can lead to differences up to 138% for the same variable. It is also found that the limits for the use of wave theories depends on the particular model and that the range of applicability of different theories can be extended.