When using PrintDocument class for printing on vb.net, It won’t allow you to change PaperSize Width and Height properties.

In order to set those values manually, first, you need to define a custom paper size using the code below. Then, you can set it to PrintDocument’s PaperSize property:

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

I hope this helps you with your programming, Cheers!