使用Graphics.MeasureString()
来自MSDN：http://msdn.microsoft.com/en-us/library/6xe5hazb.aspx

private void MeasureStringMin(PaintEventArgs e)
{
    // Set up string. 
    string measureString = "Measure String";
    Font stringFont = new Font("Arial", 16);

    // Measure string.
    SizeF stringSize = new SizeF();
    stringSize = e.Graphics.MeasureString(measureString, stringFont);

    // Draw rectangle representing size of string.
    e.Graphics.DrawRectangle(new Pen(Color.Red, 1), 0.0F, 0.0F, stringSize.Width, stringSize.Height);

    // Draw string to screen.
    e.Graphics.DrawString(measureString, stringFont, Brushes.Black, new PointF(0, 0));
}

如果您不想处理Paint事件处理程序，可以尝试TextRenderer类。它有一个静态方法，与上面答案中的MeasureString()方法相同。在这个类中，它被称为MeasureText。