Define a circle extent using the circle tool - arcgis javascript api

I have the following script for drawing a circle and I want to limit the user to a predefined radius:

$('#circle').click(function () {
       clearPreviousSearch();
       map.disableMapNavigation();
       $('#run').fadeOut();
       buttonclickvalue = 'circle';
       drawToolbar.activate(esri.toolbars.Draw.CIRCLE);
         });    

Where should I specify that I want to define the specific radius using the ArcGIS API script?:

var symbol = new SimpleFillSymbol().setColor(null).outline.setColor("blue");
       var gl = new GraphicsLayer({ id: "circles" });
       var geodesic = dom.byId("geodesic");
       map.addLayer(gl);
       map.on("click", function(e) {
         var radius = map.extent.getWidth() / 100;
         var circle = new Circle({
           center: e.mapPoint,
           geodesic: domAttr.get(geodesic, "checked"),
           radius: radius
           });

Any help or pointers appreciated!