Ultra-wideband Ground Penetrating Radars (GPR) are sensitive to a large range of scatterer sizes. Considering fractal heterogeneities in the subsurface, we propose a method to retrieve their typical size L. The determination of L with this method does not require a priori knowledge of the statistical distribution of permittivity values in the investigated subsurface. The method relies on the analysis of the backscattered signal by frequency/wavelength sub-bands. It is adapted to WISDOM, the GPR onboard the rover of the Rosalind Franklin ExoMars mission (ESA), but can be applied to any ultra-wideband GPR. Based on numerical simulations, a maximum in volume backscattering is reached at the wavelength (in the subsurface) . We demonstrate that this maximum, and therefore L, can be identified even in presence of moderate electrical losses, compatible with conditions expected on the Moon or Mars. Assuming an average permittivity of 5, WISDOM (0.5-3 GHz) data products could be used to estimate L as long as it is in the range 0.9-4.2 cm The retrieval method for L is validated on experimental WISDOM data acquired in a controlled environment.