There is a lot of envelope sizes used all over the world, so it possible iTextSharp has no predefined constants for them...
The PageSize.A4.Rotate() method returns a Rectangle, so you can create a new Rectangle with your own desired size and pass it to Document...
Rectangel rect = new Rectangle(x1, y1, x2, y2, 90);
Document doc = new Documnet(rect, m1, m2, m3, m4);