[away3d] 3d object -> 2d sprite

[savagelook]

How do I get the 2d sprite for a given 3d object?  For example, if I
want to know the height and width of a sphere relative to the screen
(not its 3d height and width), how do I get that?  I guess I'm looking
for access to a Sprite ideally, but a bounding box would be useful as
well.