Menu: View→Object Properties→Colors→Line width
Specifies the width (in units of 1/96 inch) of lines used to draw rectangles, ellipses and arrows. Leave the line width blank in the Object Properties dialog if you want to use the default line width specified on the Pen Width tab of the Interface Properties dialog. See To specify the line width (pen width).
See also:
To change the line width (pen width) for a single object
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("", "PenwidthCombo")
aComboBox.SelectedIndex = 0