@aparnabn,
The unit of FixHeight and FixWidth properties is the point (72 pints = 1 inch, 1 inch = 25.4 mm). This is the code example to add an image in the PDF document:
C#
string dataDir = @"C:\Pdf\test627\";
// Instantiate Document object
Document doc = new Document();
// add page to pages collection of PDF file
Aspose.Pdf.Page page = doc.Pages.Add();
// Create an image instance
Aspose.Pdf.Image img = new Aspose.Pdf.Image();
img.FixWidth = 42.51; // 15mm = (15/25.4*72) points
img.FixHeight = 600;
// Set image type as SVG
img.FileType = Aspose.Pdf.ImageFileType.Unknown;
// Path for source file
img.File = dataDir + "Example.png";
page.Paragraphs.Add(img);
//Set page properties
page.PageInfo.Width = 800;
page.PageInfo.Height = 800;
// save resultant PDF file
doc.Save(dataDir + "Test.pdf");