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.