While using PrintDocument class for printing on vb.net, It won’t allow you to change PaperSize Width and Height values. In order to set those values manually, you need to define a custom paper size first. Use the code below to create a new custom paper size and then set it to PrintDocument’s PaperSize property:

Dim pkCustomSize1 As New System.Drawing.Printing.PaperSize("Custom Paper Size", 800, 600)
PrintDocument1.PrinterSettings.DefaultPageSettings.PaperSize = pkCustomSize1

I hope this helps, cheers!