dataGrid1.TableStyles["A"].GridColumnStyles[col].Width = width;
下面是一个自动设值列宽的函数，可以参考一下：
/// autosize the first column
/// </summary>
/// <param name="col">column number</param>
private void AutoSizeCol(int col)
{
float width = 0;
int numRows = ((DataTable) dataGrid1.DataSource).Rows.Count;
Graphics g = Graphics.FromHwnd(dataGrid1.Handle);
StringFormat sf = new StringFormat(StringFormat.GenericTypographic);
SizeF size;
for(int i = 0; i < numRows; ++ i)
{
size = g.MeasureString(dataGrid1[i, col].ToString(), dataGrid1.Font, 500, sf);
if(size.Width > width)
width = size.Width;
}
g.Dispose();
dataGrid1.TableStyles["A"].GridColumnStyles[col].Width = (int) width + 16;
}