Try the below code to set the font size of the data label.
DataLabelEx lbl = new DataLabelEx(series);
lbl.ShowSeriesName = true;
lbl.Id = 0;
PortionFormatEx pt = lbl.TextProperties.Paragraphs[0].ParagraphFormat.DefaultPortionFormat;
pt.FontHeight = 8;
series.Labels.Add(lbl);