Mn doped ZnO with different percentages of Mn content (1- 5 mol %) were prepared by a simple sol-gel method. The band gaps of the materials were systematically studied based on the dependence of the dimensions of the nanostructures as well as the effects of increasing Mn content in ZnO materials. It was found that by increasing Mn content in ZnO nanostructures leads to band gap widening with respect to the pure ZnO materials. In order to understand the phenomenon of band gap changes, quantitative X-Ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS) and microscopy were employed. It was found that band gaps of these materials were affected by both the dimensions of the nanostructures and the presence of the dopant element. Changes in the dimensions of the crystallites affected the crystal structural parameters which in turn affected the band gaps of the materials. Band gap changes for Mn doped ZnO materials are not the same for nano and micro crystallites. XPS shed some light on the band gap changes of the materials.