Using "fixedColumnWidth"

HTML

<script type="text/javascript" src="http://www.amcharts.com/lib/3/amcharts.js"></script>
<script type="text/javascript" src="http://www.amcharts.com/lib/3/serial.js"></script>
<script type="text/javascript" src="http://www.amcharts.com/lib/3/themes/none.js"></script>
<div id="chartdiv" style="display: none;"></div>

CSS

#chartdiv {
	width		: 100%;
	height		: 500px;
	font-size	: 11px;
}

JavaScript

// this will get executed BEFORE chart is drawn
// we'll use it to modify the width of the chart area
AmCharts.addInitHandler(function(chart) {
    // total offsets reserved for margins
    var offsets = 100;
    
    // pixel value reserved for each category
    var categoryWidth = 80;
    
    // calculate width
    var width = offsets + chart.dataProvider.length * categoryWidth;
    
    // set container width
    document.getElementById("chartdiv").style.width = "" + width + "px";
    
    // since the chart is already build for the initial container size we need to
    // revalidate it's size. we'll delay a little bit the call to invalidateSize()
    // so that the chart elements are created first
    setTimeout(function () {
        chart.invalidateSize();
        document.getElementById("chartdiv").style.display = "block";
    }, 1);
}, ['serial']);

var chart = AmCharts.makeChart("chartdiv", {
    "type": "serial",
    "theme": "none",
    "dataProvider": [{
        "country": "USA",
        "visits": 20250
    }, {
        "country": "China",
        "visits": 18820
    }, {
        "country": "China",
        "visits": 15
    }],
    "valueAxes": [{
        "gridColor":"#FFFFFF",
        "gridAlpha": 0.2,
        "dashLength": 0
    }],
    "gridAboveGraphs": true,
    "startDuration": 1,
    "graphs": [{
        "balloonText": "[[category]]: <b>[[value]]</b>",
        "fillAlphas": 0.8,
        "lineAlpha": 0.2,
        "type": "column",
        "valueField": "visits"
    }],
    "chartCursor": {
        "categoryBalloonEnabled": false,
        "cursorAlpha": 0,
        "zoomable": false
    },
    "categoryField": "country",
    "categoryAxis": {
        "gridPosition": "start",
        "gridAlpha": 0,
         "tickPosition":"start",
         "tickLength":20
    },
    "exportConfig":{
      "menuTop": 0,
      "menuItems": [{
      "icon": '/lib/3/images/export.png',
      "format": 'png'     
      }]  
    }
});