Hello Andrew

I suppose you could use one of "fontSize" Highcharts API in a wpDataCharts callback. 

I never tested it, so I don't know which one you would need to use.

Hi again Andrew

I can't say for sure, as I haven't changed the font size of charts using the callbacks.

Maybe you can try with something like this:

<script type="text/javascript">
jQuery(window).on('load', function() {
    if( typeof wpDataChartsCallbacks == 'undefined' ){ wpDataChartsCallbacks = {}; }
    wpDataChartsCallbacks[39] = function(obj) {
    obj.options.xAxis[0] = {
           labels: {
             style: {
                fontSize: '20px'
             }
           }
        }
    }
});
</script>

Something like this can be achieved here, if you change:

color: 'red'

to:

fontSize: '20px'

So, it's a good place to look.