I am not sure if I understand your question correctly. Therefore, here is another sample I created for you, which puts a rectangle/polygon on the mouse click location. The polygon size is predefined.Here is the sample:<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<meta http-equiv="X-UA-Compatible" content="IE=7,IE=9" />
<meta name="viewport" content="initial-scale=1, maximum-scale=1,user-scalable=no"/>
<title>Add graphics</title>
<link rel="stylesheet" type="text/css" href="http://serverapi.arcgisonline.com/jsapi/arcgis/2.4/js/dojo/dijit/themes/claro/claro.css">
<script type="text/javascript" src="http://serverapi.arcgisonline.com/jsapi/arcgis/?v=2.4"></script>
<script type="text/javascript">
dojo.require("esri.map");
var map;
function init() {
map = new esri.Map("map");
var tiledMapServiceLayer = new esri.layers.ArcGISTiledMapServiceLayer("http://server.arcgisonline.com/ArcGIS/rest/services/ESRI_StreetMap_World_2D/MapServer");
map.addLayer(tiledMapServiceLayer);
}
function createGraphic() {
dojo.connect(map, "onClick", addGraphic);
}
function addGraphic(evt) {
var polygon = new esri.geometry.Polygon(new esri.SpatialReference({wkid:4326}));
var deltaX = 10;
var deltaY= 5;
var xmin = evt.mapPoint.x - deltaX;
var xmax = evt.mapPoint.x + deltaX;
var ymin = evt.mapPoint.y -deltaY;
var ymax = evt.mapPoint.y + deltaY;
var polygon = new esri.geometry.Polygon(new esri.SpatialReference({wkid:4326}));
polygon.addRing([[xmin,ymin],[xmin,ymax],[xmax,ymax],[xmax,ymin],[xmin,ymin]]);
var sfs = new esri.symbol.SimpleFillSymbol(esri.symbol.SimpleFillSymbol.STYLE_SOLID,
new esri.symbol.SimpleLineSymbol(esri.symbol.SimpleLineSymbol.STYLE_DASHDOT,
new dojo.Color([255,0,0]), 2),new dojo.Color([255,255,0,0.25]));
var graphic = new esri.Graphic(polygon, sfs);
map.graphics.add(graphic);
alert("Rectangle size is:" + deltaX + " x " + deltaY);
}
dojo.addOnLoad(init);
</script>
</head>
<body class="tundra">
<button onclick="createGraphic()">Polygon</button>
<div id="map" style="width:900px; height:600px; border:1px solid #000;"></div>
</body>
</html>