// Prepare HTML code
var code = @"<style>
    div { page-break-after: always; }
</style>
<div style='border: 1px solid red; width: 300px'>First Page</div>
<div style='border: 1px solid red; width: 500px'>Second Page</div>";

// Initialize an HTML document from the HTML code
using var document = new HTMLDocument(code, ".");

// Create the instance of Rendering Options and set a custom page-size
var options = new PdfRenderingOptions();
options.PageSetup.AnyPage = new Page(new Size(400, 200));

// Enable auto-adjusting for the page size
options.PageSetup.AdjustToWidestPage = true;

// Prepare a path to save the converted file 
string savePath = Path.Combine(OutputDir, "output-widest-page-size.pdf");

// Create the PdfDevice and specify options and output file
using var device = new PdfDevice(options, savePath);

// Render HTML to PDF
document.RenderTo(device);