Refer to "thisContent" in a shape layer content expression?

sourceRectAtTime() will look at the total area of all of the elements on a shape layer and return the width, height, top, and left values for all of the shapes. 

A parametric shape like a rectangle or ellipse will have size values you can use, but a shape drawn with a path does not. As long as a property can have a keyframe applied to it you can use that value in an expression. 

If all you want to do is modify the scale of a rectangle and have that change the size or position you can just so some simple math using expressions. If you are new to expressions let us know. It takes a while to figure out how they work. 

If you can provide some specific design goals for your project we can give you a better idea. I use expressions all the time to modify the size or position of layers based on scale or distance from a camera. Let us know exactly what you want to do and we can probably help.