public SizeF MeasureString(
    string text,
    DXFont font
)

Public Function MeasureString(
    text As String,
    font As DXFont
) As SizeF

using DevExpress.Pdf;
using System.Drawing;
using DevExpress.Drawing;
//...

using (var processor = new PdfDocumentProcessor())
{
    processor.CreateEmptyDocument();
    using (PdfGraphics graphics = processor.CreateGraphicsWorldSystem())
    {
        // Obtain the first document page
        PdfPage page = processor.AddNewPage(PdfPaperSize.A4);
        PdfRectangle pageSize = page.CropBox;

        // Specify text to draw
        string text = "PDF Document API";
        using (var textBrush = new DXSolidBrush(Color.FromArgb(255, Color.DarkOrange)))
        {
            DXFont font = new DXFont("Segoe UI", 20, DXFontStyle.Regular))
            // Calculate text size
            SizeF textSize = graphics.MeasureString(text, font);

            // Calculate a point where to draw text
            PointF textPoint =
                new PointF((float)((pageSize.Width - textSize.Width) / 2), (float)((pageSize.Height - textSize.Height) / 2));

            // Draw text at the calculated point
            graphics.DrawString(text, font, textBrush, textPoint);

            // Add graphics content to the page foreground
            graphics.AddToPageForeground(page);
        }
    }
    processor.SaveDocument("result.pdf");
}

Imports DevExpress.Pdf
Imports System.Drawing
Imports DevExpress.Drawing
'...

Using processor = New PdfDocumentProcessor()
  processor.CreateEmptyDocument()

  Using graphics As PdfGraphics = processor.CreateGraphicsWorldSystem()
    ' Obtain the first document page
    Dim page As PdfPage = processor.AddNewPage(PdfPaperSize.A4)
    Dim pageSize As PdfRectangle = page.CropBox

    ' Specify text to draw
    Dim text As String = "PDF Document API"
    Using textBrush As New DXSolidBrush(Color.FromArgb(255, Color.DarkOrange))
      Dim font As New DXFont("Segoe UI", 20, DXFontStyle.Regular)
      ' Calculate text size
      Dim textSize As SizeF = graphics.MeasureString(text, font)

      ' Calculate a point where to draw text
      Dim textPoint As New PointF(CSng((pageSize.Width - textSize.Width) \ 2), CSng((pageSize.Height - textSize.Height) \ 2))

      ' Draw text at the calculated point
      graphics.DrawString(text, font, textBrush, textPoint)

      ' Add graphics content to the page foreground
      graphics.AddToPageForeground(page)
    End Using
  End Using
  processor.SaveDocument("result.pdf")
End Using

using (PdfGraphics graphics = documentProcessor.CreateGraphicsPageSystem()) {
    SizeF stringSize = graphics.MeasureString(text,font);
    float scale = (float)(watermarkSize / (double)stringSize.Width);