In Flash, you can scale a “symbol” (like exported canvas) by pulling handles on the sides of a bounding box. It looks like the closest thing to that is the Warp Layer, so is there a way to automatically fit the Warp Layer’s source rectangle around an object?
Sorry for your time 
. I know there is a Stretch layer, but I like to be able to see the bounds of the object.
Not possible. Warp layer works in a raster world. It means that it looks the layers below once they have been rendered. If it needs more precision it asks for a deep pixel subsampling but it doesn’t know nothing about the geometry of the layers below.
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Wait, there is one information that might be used somehow. There is one value used to render a boundary box around the selected layers I think it can be used for other layers to fit on the warp rectangle. It has one problem. If the layers below the warp layer changes its bounding rectangle (i.e. they get wider) then the warp layer would modify its render result if the warp rectangle follows this bounding rectangle.
In any case, currently there is not API to do that connection between the Warp layer and its context layers boundaries. It has to be coded in any case.
-G