[Solved]-Highcharts: Marker radius does not seem to work-Highcharts

In HighStock, unlike HighCharts, the default for marker is enabled: false, this is because the data tends to be very dense and markers wouldn't add much value to the data comprehension of the user.

You need to modify your code to:

plotOptions: {
    series: {
        marker: {
            enabled: true,
            radius: 90
        }
    }
},

for the markers to show up.

EDIT: I will leave the above up in case someone comes across this and needs it. What David really wants to know is whether the series symbol during hover can be changed. The code for that is:

plotOptions: {
            series: {
                marker: {
                    lineColor: null,
                    states: {
                        hover: {
                            fillColor: 'white',
                            radius: 10
                        }
                    }
                }
            }
        }

Which is straight from the Highcharts API reference at http://api.highcharts.com/highcharts#plotOptions.scatter.marker.states.hover.radius