We present a comprehensive study of magnetization reversal process in thin films of Mn5Ge3. For this investigation, we have studied the magnetic anisotropy of Mn5Ge3 layers as a function of the film thickness using VSM and SQUID magnetometers. The samples grown by molecular beam epitaxy exhibit a reorientational transition of the easy axis of magnetization from in-plane to out-of-plane as the film thickness increases. We provide evidences that above a critical thickness estimated to 20 nm, the magnetic structure is constituted of stripes with out-of-plane magnetization pointing alternatively up and down. We have analyzed our results using different phenomenological models and all the calculations converge towards values for magnetocrystalline anisotropy constant and saturation magnetization that are in excellent agreement with the reported values for bulk Mn5Ge3. This study has also lead to the first estimation in Mn5Ge3 of the exchange constant, the surface energy of domain walls as well as their width. These parameters are essential for determining whether this material can be used in the next generation of spintronic devices.