Set your legend Alignment in bottom for Avoiding width increment of chart as below.
Chart.GetLegend().SetAlignment(laBottom);
if you want custom position of legend (Right side of chart)then you can do as below.
m_Chart1.GetLegend().SetCustomPosition(TRUE);
m_Chart1.GetLegend().SetPositionUnits(muPercent);
m_Chart1.GetLegend().SetTopPercent(20);
long leftmargin = 80.0;
m_Chart1.GetLegend().SetLeftPercent(leftmargin );
m_Chart1.GetPanel().SetMarginRight(100-leftmargin+1);//for avoiding overlapping of legend and chart
Also you can finetune your legend and chart by modifying above value