Hi,
Thank you for using Syncfusion Software.
Please use the below code snippet to set predefined and custom page size
PdfDocument doc = new PdfDocument();
//To set predefine page size
doc.PageSettings.Size = PdfPageSize.B2;
//To set custom page size
doc.PageSettings.Size = new SizeF(400, 400);
doc.Pages.Add();
Also, in Essential PDF everything is Point based, so please specify the units in Points. For more information regarding PDF units please refer the below URL.
http://help.syncfusion.com/ug_72/pdf/SetGraphicUnits.htmlBest Regards,
Bhuvana