Odd-sized siege weapons are buggy.  For example, take a medium character, set the weapon size to Small, and click to add a new weapon.   Click "Ballista, heavy".   The right pane on the selection screen will show "Class: 2-Hand" and "Damage: 2d10".  Click "Add".   In the weapon list the damage will now show as "4d8".  Click the "?" by the weapon and it will show "Class: Siege" and "Damage: 4d8".
It seems there's no way to actually add an odd-size siege weapon.
				
			It seems there's no way to actually add an odd-size siege weapon.
 
	 There's a LOT to keep straight with Alchemists, but they're fun to play.
 There's a LOT to keep straight with Alchemists, but they're fun to play.