Try setting my.min_x my.max_x my.min_y my.max_y to values you can read in MED or just play around with, say, my.min_x*=2; my.max_x*=2; my.min_y*=2; my.max_y*=2;
in the action attached to your model.
If you've got an A6.3 version, I guess there's an instruction like set_hull or correct_collisionbox or something, just search the help file for the "collision" entry.