In the cave south of Hitano in Sky World, the hole near the entrance is "too small to fit some heads." So how do you know what kinds of monsters are small enough to fit? The library in GreatLog can help you. When you look up a species, you'll see its skills, descriptions, rarity, etc.... and also its size. So to fit through the hole, just make sure that your entire party, including the monster master, are size S.