We address the line shape and line widths observed in recent inelastic electron tunneling spectroscopy (IETS) experiments. The nonequilibrium Green function (NEGF) formalism is used to analyze the effect of the electron-phonon interaction on the tunneling spectra. We find that IETS line shapes are sensitive to junction parameters, in particular the position of the bridge electronic resonance and the molecule-lead coupling that may be controlled experimentally. Intrinsic IETS line widths are found to be dominated by the coupling of molecular vibrations to electron-hole pairs excitations in the lead(s) to which the molecule is bonded chemically. While estimated widths are of similar order of magnitude as observed in the recent experiment of Wang et al. (Nano Lett. 2004, 4, 643), one cannot rule out inhomogeneous contribution to the line width in this monolayer experiment.