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.