From the RuleBook:
"Slashing, piercing, or bludgeoning weapon more than 18 inches up to 36 inches in length. At least
2/3 of its length must be strike-legal."
Certain classes use a short sword as a primary weapon. However, all classes can use a short sword, even if it not the primary weapon. If a Raider is using a Long Sword, he is in violation of the rules. He must retrieve a short sword, forgo the long sword, or sit out the rest of that battle.