Hi John,
You have to touch the MarkSize as well as the series legend text style:
NChart chart = nChartControl1.Charts[0];
NBarSeries bar = new NBarSeries();
bar.Values.Add(10);
bar.Values.Add(20);
bar.Legend.TextStyle.FontStyle.EmSize = new NLength(8);
chart.Series.Add(bar);
NLegend legend = nChartControl1.Legends[0];
legend.Data.MarkSize = new NSizeL(5, 5);
Hope this helps - let me know if you have any questions or meet any problems.
Best regards,
Bob