We present first 3D micromagnetic calculations on finite as well as infinite square prisms. We have studied the magnetization reversal mechanisms taking place in these systems as a function of the prism width. We observe strong differences between reversal mechanisms in infinite prisms and nucleation/reversal processes in finite prisms. For infinite prisms buckling and curling are the modes that have been obtained in the whole width range studied. Substantial differences have been observed between magnetization reversal in finite prisms and inifinite prisms. While for small widths the magnetization reversal takes place following a buckling-like deformation strongly influenced by the extremities, for large widths, domain nucleation and wall propagation has been found to be the ad hoc reversal mechanism after a curling like nucleation. From stability arguments we have determined the critical domain (or activation volume) in low anisotropy systems. Both the switching fields and the activation volume obtained from simulations compare favorably with measurements on Ni electrodeposited nanowires.