Flash MX 2004.
stroke.variation
Property; a string that specifies the variation of a stippled line. This property is available only if the stroke.style property is set to stipple (see stroke.style). Acceptable values are "one size", "small variation", "varied sizes", and "random sizes".
The following example sets the variation property to random sizes for a stroke style of stipple:
var myStroke = fl.getDocumentDOM().getCustomStroke();
myStroke.style = "stipple";
myStroke.dotSpace= 3;
myStroke.variation = "random sizes";
myStroke.density = "sparse";
fl.getDocumentDOM().setCustomStroke(myStroke);