Hello,The units in the Ox dynamic simulations depend on your scene units, I suppose the general rule of thumb is not to make objects to big or small. There are some good rules about using dynamics outlined in the dynamics video tutorials:
http://www.youtube.com/watch?v=h0U0rC53qXQ
Michael is on vacation currently but I'll ask him to reply once he's back.
Marsel Khadiyev (Software Developer, EPHERE Inc.)