So when I was researching a stealth-based ninja I came across weapon finesse and piranha strike which mention light weapons. I know there's the light weapon property but what would the criteria for light weapons be assuming their not on the SRD but exist in IRL.
You are trained in using your agility in melee combat, as opposed to brute strength.
Benefit: With a light weapon, elven curve blade, rapier, whip, or spiked chain made for a creature of your size category, you may use your Dexterity modifier instead of your Strength modifier on attack rolls. If you carry a shield, its armor check penalty applies to your attack rolls.
Special: Natural weapons are considered light weapons.
You make a combination of quick strikes, sacrificing accuracy for multiple, minor wounds that prove exceptionally deadly.
Prerequisites: Weapon Finesse, base attack bonus +1.
Benefit: When wielding a light weapon, you can choose to take a -1 penalty on all melee attack rolls and combat maneuver checks to gain a +2 bonus on all melee damage rolls. This bonus to damage is halved (-50%) if you are making an attack with an off-hand weapon or secondary natural weapon. When your base attack bonus reaches +4, and for every 4 points thereafter, the penalty increases by -1 and the bonus on damage rolls increases by +2. You must choose to use this feat before the attack roll, and its effects last until your next turn. The bonus damage does not apply to touch attacks or effects that do not deal hit point damage. This feat cannot be used in conjunction with the Power Attack feat.