chart js 2 how to set bar width

For version 2.8+ (and apparently as far back as 2.2), there are now some excellent controls over the bar thickness, max thickness, etc.

Per the Chart.js documentation, you would set them like so:

{
    type: 'bar', // or 'horizontalBar'
    data: ...,
    options: {
        scales: {
            xAxes: [{
                barThickness: 6,  // number (pixels) or 'flex'
                maxBarThickness: 8 // number (pixels)
            }]
        }
    }
}

You were right : The attribute you have to edit is barPercentage.

But maybe the error comes from where you edited the value.

As you can see in the bar chart options :

Name : barPercentage
- Type : Number
- Default : 0.9
- Description : Percent (0-1) of the available width each bar should be within the category percentage. 1.0 will take the whole category width and put the bars right next to each other. Read More

The attribute is actually stored in scales.xAxes ("Options for xAxes" table).

So you just have to edit your chart this way :

var options = {
    scales: {
        xAxes: [{
            barPercentage: 0.4
        }]
    }
}

var data = {
    labels: ["January", "February", "March", "April", "May", "June", "July"],
    datasets: [{
        label: "My First dataset",
        backgroundColor: "rgba(75,192,192,0.4)",
        borderColor: "rgba(75,192,192,1)",
        data: [65, 59, 75, 81, 56, 55, 40],
    }]
};

var ctx = document.getElementById("myChart");
var myChart = new Chart(ctx, {
    type: 'bar',
    data: data,
    options: {
        scales: {
            yAxes: [{
                ticks: {
                    beginAtZero: true
                }
            }],
            xAxes: [{
                // Change here
            	barPercentage: 0.2
            }]
        }
    }
});

console.log(myChart);

<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.1.6/Chart.js"></script>
<canvas id="myChart"></canvas>

Update (Chart.js Version 2.2.0+)

As stated in the Release Version 2.2.0 - Candidate 2 :

Enhancements

• Can now manually configure the thickness of a bar in a bar chart. Use a new barThickness option on the correct axis to set the thickness of a bar.
• And so on ...