seriesDefaults.legendItem.markers.borderRadiusNumber

The border radius in pixels when type is set to "roundedRect".

Example

<div id="chart"></div>
<script>
$("#chart").kendoChart({
    series: [{
        type: "line",
        name: "Series A",
        data: [1, 2, 3],
        markers: { visible: true }
    }],
    seriesDefaults: {
        legendItem: {
            markers: {
                type: "roundedRect",
                borderRadius: 5
            }
        }
    }
});
</script>