In this paper, we present a method that may lead to a better quantitative insight into the relation between the size (horizontal dimensions) of a natural groundwater flow modelling area and the schematization required for a numerical model. The method is based on a decomposition of the spatial variability of the groundwater table into a Fourier series. Each sinusoidal Fourier mode has a specific wavelength that can be considered as a measure of the scale represented by the mode. Modes with small wavelengths correspond to local differences in the groundwater table and modes with large wavelengths correspond to regional differences in the groundwater table. The contribution of each individual Fourier mode to the groundwater flow at a certain depth can be determined. This offers the opportunity to evaluate which details in the variation of the groundwater table should be taken into account in a certain numerical model. A penetration depth is defined and used to determine the depth over which the groundwater flow pattern is significantly influenced by a certain Fourier mode. Such an analysis can be performed for each important Fourier mode in the problem under consideration. This approach results in simple algebraic expressions to assess grid size and the vertical dimension of a numerical model.