Just use bevel! Open the Modeling tool kit and with the edge(s) you want beveled selected, click on the bevel button. Just below the button in the bevel options you have 2 options, Segments and Offset. Select either of these to adjust what you want. Segments being how many segments on the bevel you want and offset basically being the size of the bevel itself. Just remember on the segments, you will probably have to fix the topology to keep quads in your model!
