[Solved]-Resize data points with highcharts-Highcharts

This gray line is called grid line, you can customize them following the reference.

To remove grid lines

 yAxis: {
     gridLineWidth: 0
 }

Like this example.

Change marker width

plotOptions: {
    series: {
        marker: {
            enabled: true,
            symbol: 'circle',
            radius: 7
        }
    }
}

demo

You can take a look here if you want to customize series markers.