How to find the length of the string while drawing into PDF?

// Creates a new instance of the PdfDocument class.
PdfDocument document = new PdfDocument();
// Adds a page to the document.
PdfPage page = document.Pages.Add();
// Creates PDF graphics for the page.
PdfGraphics g = page.Graphics;
// Creates a solid brush.
PdfBrush brush = new PdfSolidBrush(Color.Black);
// Sets the font.
PdfFont font = new PdfStandardFont(PdfFontFamily.Helvetica, 20f);
//String used to draw.
string lengthString = "Hello world!";
//Measures string method for finding the length.
SizeF size= font.MeasureString(lengthString);
// Draws the text.
g.DrawString(lengthString, font, brush, new PointF(20, 20));
// Saves and closes the document.
document.Save("Sample.pdf");
document.Close(true);