Menu: View→Interface Properties→Pen Width→Arrowhead Size
Select the width (in units of 1/96 inch) of arrowheads.
See: To specify the size of arrowheads
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", "NUpDownArrowHeadWidth")