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 src="//www.amcharts.com/lib/4/themes/animated.js"></script>
<div id="chartdiv"></div>

CSS

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

JavaScript

var chart = AmCharts.makeChart("chartdiv", {
    "type": "serial",
    "theme": "none",
    "dataProvider": [{
        "country": "USA",
        "visits": 2025
    }, {
        "country": "China",
        "visits": 1882
    }],
    "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",
        "fixedColumnWidth": 50
    }],
    "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'     
      }]  
    }
});