There is no easy way to access it, and it changes depending on the behavior. Also, bear in mind changing the size of a Physics object will cause it to destroy the physics body and teleport in a new one of the new size (since it's the only way it's possible in a physics engine), and that can cause glitches.
The best thing to do is have a parameter in an action where the user passes a speed. Then it will work with all behaviors and any custom coded movements too.