Frame.WidthRule (Word)

Returns or sets the rule used to determine the width of a frame. Possible return values are wdFrameAtLeast - Sets the height or width to a value equal to or greater than the value specified by the Height property or Width property, wdFrameAuto - Sets the height or width according to the height or width of the item in the frame, wdFrameExact - Sets the height or width to an exact value specified by the Height property or Width property.

If ActiveDocument.Frames.Count >= 1 Then 
 With ActiveDocument.Frames(ActiveDocument.Frames.Count) 
 .WidthRule = wdFrameExact 
 .Width = 72 
 End With 
End If