How to change a size of a page in PDF GemBox

Question

I'm trying to create a PDF from the scratch using GemBoxPdf

  using (var document = new PdfDocument())
        {
            // Add a page.
            var page = document.Pages.Add();
            page.Rotate = 90;
            // Write a text.
            using (var formattedText = new PdfFormattedText())
            {
                formattedText.Append("Hello World!");

                page.Content.DrawText(formattedText, new PdfPoint(100, 700));
            }}

I'd like to change the size of the page to be able to set it as A1,A2,A3 or A4 or custom size

I've tried to set the cropBox but it does not work. I've seen another questions of how to do it with spreedsheet but using its PrintOptions I am also unable to change its size

How do I modify the page size ?

Thanks