Menu: View→Interface Properties→Page Layout→Paper Size
You can use the drop down list to select a named paper size, such as "Portrait - A4" or "Landscape - Letter", for the path diagram. Alternatively, you can specify the paper's height and width.
See To change the default paper size
Programming
In Amos Graphics programs, you can select the first item in this ComboBox as follows (Visual Basic):
Dim aComboBox As Windows.Forms.ComboBox = pd.GetComboBox("InterfacePropertiesForm", "ComboPaperSize")
aComboBox.SelectedIndex = 0