use a field-emission SEM with X-ray detectors for EDXS so you can get an estimate of length, width, regularity of growth and average chemical composition.
You can characterize the nanowires by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), neutron diffraction (ND), and recently electron backscattered diffraction (EBSD) techniques. Noting that, these techniques (SEM, TEM and EBSD) require very fine preparations of the investigated samples.