Menu: View→Interface Properties→Pen Width→Printer Pen Width
Select the width (in units of 1/96 inch) of lines used to draw rectangles, ellipses and arrows.
See To specify the default line width (pen width).
Programming
In Amos Graphics programs, you can obtain a reference to this NumericUpDown control as follows (Visual Basic):
Dim x As Windows.Forms.NumericUpDown = pd.GetNumericUpDown("InterfacePropertiesForm", "NUpDownPenWidth")