Chart.js axes label font size

The fontSize attribute is actually in scales.xAxes.ticks and not in scales.xAxes as you thought.

So you just have to edit the attribute like this :

var options = {
    scales: {
        yAxes: [{
            ticks: {
                fontSize: 40
            }
        }]
    }
}

You can see a fully working example in this jsFiddle and here is its result :