Typically, a circle marker symbol is used to symbolize a point, not an area.
When working in a projected coordinated system that uses meters, you can use the following to get the width of a pixel in meters:
var pxWidth = map.extent.getWidth() / map.width;
While the geometry service provides a more robust implementation, you can also draw circles client side with a center point and a radius. Here's a simple example:
<!doctype html>
<html lang="en">
<head>
<meta 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></title>
<link rel="stylesheet" href="http://serverapi.arcgisonline.com/jsapi/arcgis/2.5/js/dojo/dijit/themes/tundra/tundra.css">
<style>
html, body { height: 100%; width: 100%; margin: 0; padding: 0; }
#map{ margin: 0; padding: 0; }
#controls, #feedback {
position: absolute;
height: 80px;
font-family: arial;
bottom: 10px;
margin: 5px;
padding: 5px;
z-index: 40;
background: #fff;
color: #444;
-moz-box-shadow: 0 0 5px #888;
-webkit-box-shadow: 0 0 5px #888;
box-shadow: 0 0 5px #888;
}
#controls {
width: 440px;
left: 10px;
}
#feedback {
width: 200px;
right: 10px;
}
h3 { margin: 0 0 5px 0; border-bottom: 1px solid #444; }
.label { display: inline-block; width: 140px; }
</style>
<script>var dojoConfig = { parseOnLoad: true };</script>
<script src="http://serverapi.arcgisonline.com/jsapi/arcgis/?v=2.5"></script>
<script>
dojo.require("dijit.layout.BorderContainer");
dojo.require("dijit.layout.ContentPane");
dojo.require("esri.map");
var map, sym, radius = 1000000;
// radius is in meters since the map is in web mercator
function init() {
var ext = new esri.geometry.Extent({"xmin":-5042425,"ymin":3604239,"xmax":8048485,"ymax":8584264,"spatialReference":{"wkid":102100}});
map = new esri.Map("map",{ extent: ext });
var basemap = new esri.layers.ArcGISTiledMapServiceLayer("http://server.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer");
map.addLayer(basemap);
sym = new esri.symbol.SimpleFillSymbol().setColor(new dojo.Color([180, 0, 180, 0.25]));
dojo.connect(map, "onLoad", function() {
dojo.connect(dijit.byId("map"), "resize", map, map.resize);
dojo.connect(map, "onClick", addCircle);
});
}
function addCircle(e) {
console.log("clicked the map: ", e);
var pt, radius, circle, ring, pts, angle;
pt = e.mapPoint;
circle = new esri.geometry.Polygon(map.spatialReference);
ring = []; // point that make up the circle
pts = 40; // number of points on the circle
angle = 360 / pts; // used to compute points on the circle
for(var i = 1; i <= pts; i++) {
// convert angle to raidans
var radians = i * angle * Math.PI / 180;
// add point to the circle
ring.push([pt.x + radius * Math.cos(radians), pt.y + radius * Math.sin(radians)]);
}
ring.push(ring[0]); // start point needs to == end point
circle.addRing(ring);
map.graphics.add(new esri.Graphic(circle, sym));
console.log("added a graphic");
}
dojo.ready(init);
</script>
</head>
<body class="tundra">
<div data-dojo-type="dijit.layout.BorderContainer"
data-dojo-props="design:'headline',gutters:false"
style="width: 100%; height: 100%; margin: 0;">
<div id="map"
data-dojo-type="dijit.layout.ContentPane"
data-dojo-props="region:'center'">
<div id="feedback">
Click the map to add circles.
</div>
</div>
</div>
</body>
</html>
