The PdfSharp.Drawing.XGraphics object has a MeasureString method that returns what you require.
var pdfDoc = new PdfSharp.Pdf.PdfDocument();
var pdfPage = pdfDoc.AddPage();
var pdfGfx = PdfSharp.Drawing.XGraphics.FromPdfPage(pdfPage);
var pdfFont = new PdfSharp.Drawing.XFont("Helvetica", 20);
while (pdfGfx.MeasureString("Hello World!").Width > pdfPage.Width)
--pdfFont.Size;
pdfGfx.DrawString("Hello World!", pdfFont
, PdfSharp.Drawing.XBrushes.Black
, new PdfSharp.Drawing.XPoint(100, 100));
This should help you. Please consider that I didn't test this code as I wrote it on the fly in order to help. It might contain some compile-time errors, but you may get the idea.