So far I came to the conclusion that to do what I wan ti Need:
1º Create a new creature with the stats of a bear but with different size.
2º Create a new script that just like sumon animal companion but that points to this "modified bear" instead
3º Modify the sumon animal companion so that it uses the new script.
Because I think if I just change the appearance of the bear/summoned creatures it will change the appearance of other monster in the game that use the same model.
If anyone could point to me if I'm in the right direction would be a big help. Also, I don't know where to find the file that contains the scripts.