In order to make a diagram of a specific size both on screen and when printing you should perform the following steps (I’m using the A5 paper size here, because my printer does not support A3):
1. Change the drawing document measurement unit and configure the document’s size
document.MeasurementUnit = NMetricUnit.Millimeter;
document.Width = 210;
document.Height = 148;
2. Configure the page settings of the drawing document, make sure to set the printing margins to 0
NPageSettings pageSettings = document.Settings.PageSettings;
pageSettings.Landscape = true;
pageSettings.PaperKind = PaperKind.A5;
pageSettings.MarginsLeft = 0;
pageSettings.MarginsTop = 0;
pageSettings.MarginsRight = 0;
pageSettings.MarginsBottom = 0;
3. When needed, create a print manager and configure it to print the document on a single page
NPrintManager printManager = new NPrintManager(document);
printManager.Layout = PagedLayout.FitToPages;
printManager.PageRows = 1;
printManager.PageColumns = 1;
printManager.ShowPrintPreview();
If you want to skip the print preview dialog, use the Print method instead of the ShowPrintPreview one.