In AS3.0 it is like: mc.width and mc.height
In AS2.0 it is like: mc._width and mc._height
What about createJs mc.????
If the MC was generated by Flash then you can use either myMC.nominalBounds, which returns the first frame's bounds, or myMC.getBounds() which returns the current frame bounds. These values will represent the author-time dimensions, and will not reflect changes you made to the contents of the MC in code.
If the MC was not generated by Flash, then you can still call getBounds(), but it will not include bounds for Shape instances (unless you have set bounds on them with setBounds()).