protected override Size GetPreferredSize (Graphics graphics, DataGridViewCellStyle cellStyle, int rowIndex, Size constraintSize)
{
object o = FormattedValue;
if (o != null) {
Size s = DataGridViewCell.MeasureTextSize (graphics, o.ToString (), cellStyle.Font, TextFormatFlags.Default);
s.Height = Math.Max (s.Height, 20);
s.Width += 2;
return s;
} else
return new Size (21, 20);
}