Percent width and height are based on the percent of their parent container's size. Thus, with your example, you'd want:
var graphLayout:Canvas = new Canvas();
graphLayout.percentWidth=50;
graphLayout.percentHeight=50;
testlayout.addChild(graphLayout);
You could also use constraints, although you have to use a special
method for those, and percents seems to be more along the lines of what
you're looking for anyway.
~Nate
