Unarmed Monk Damage

Looking through the Help Files: in the monk character table (Characters/Classes/Monk) can be read that, at level 18, the unarmed damage for a medium monk becomes 3d8. But in the Weapon Damage table corresponding to the size (Characters/Basic Statistics/Size) there is no entry for 3d8 damage for a medium creature. What would be the numbers progression for the different sizes?