Trying to throw a large harpoon by a medium character?

Question

A harpoon is an exotic two handed weapon that deals 1d8 when the medium size. If a medium PC was to pick up an enemies large harpoon to throw and was proficient with harpoons,

• would it be possible as its a two handed medium weapon but now large?
• would it count as an improvised weapon due to its size?
• what would the damage be (since its a harpoon it should use the same damage)?
• what would the critical damage be (harpoons are x3 but improvised are x2)?
• since the weapon isnt the correct size for the user, it would be -2 to the attack, but under the weapon size rules, if the weapon now counts as improvised (without the throw anything feat) you are not proficient which means another -4.

Answer 1

Medium creatures cannot wield Large 2-handed weapons...

...without an ability that says otherwise. The rules on Inappropriately Sized Weapons include

Inappropriately Sized Weapons: A creature can’t make optimum use of a weapon that isn’t properly sized for it. A cumulative –2 penalty applies on attack rolls for each size category of difference between the size of its intended wielder and the size of its actual wielder. If the creature isn’t proficient with the weapon, a –4 nonproficiency penalty also applies.

The measure of how much effort it takes to use a weapon (whether the weapon is designated as a light, one-handed, or two-handed weapon for a particular wielder) is altered by one step for each size category of difference between the wielder’s size and the size of the creature for which the weapon was designed. For example, a Small creature would wield a Medium one-handed weapon as a two-handed weapon. If a weapon’s designation would be changed to something other than light, one-handed, or two-handed by this alteration, the creature can’t wield the weapon at all.

A Large one-handed weapon would be a two-handed weapon with a -2 penalty for a Medium creature (this is often how the iconic Barbarian Amiri is depicted). A Large two-handed weapon would be one weapon size greater and unwieldable.

It is further established (on the weapons page linked above) that throwing a weapon (regardless of if it has the Thrown property) counts as wielding it, with all the benefits and restrictions. It also has a rule applicable to throwing a weapon two-handed.

Thrown Weapons: Daggers, clubs, shortspears, spears, darts, javelins, throwing axes, light hammers, tridents, shuriken, and nets are thrown weapons. The wielder applies his Strength modifier to damage dealt by thrown weapons (except for splash weapons). It is possible to throw a weapon that isn’t designed to be thrown (that is, a melee weapon that doesn’t have a numeric entry in the Range column on Table 6–4), and a character who does so takes a –4 penalty on the attack roll. Throwing a light or one-handed weapon is a standard action, while throwing a two-handed weapon is a full-round action. Regardless of the type of weapon, such an attack scores a threat only on a natural roll of 20 and deals double damage on a critical hit. Such a weapon has a range increment of 10 feet.

---

There are classes that get around this with archetypes. Notably, the Barbarian Titan Mauler gets the ability to do so at level 3 and the Titan Fighter does so at level 1.

If you wanted to homerule these in to make a cool moment, you could borrow one of those abilities' penalties and apply some other 'cost' (perhaps Fatigue after the fight or Strength damage from a stressed muscle) to the character later to establish that doing what they did was not something they could typically do.

Answer 2

Yes, you can throw a weapon with two hands—it takes a full-round action for some reason, but you can.¹ And it is not improvised—it’s a weapon, being used as intended, so it remains that weapon. If you tried to smack someone with the butt end of the harpoon, that would be an improvised usage, but here you’re trying to use it as it was made to be used.

So in the end, it’s exactly like making the same attack with a Medium harpoon with only these exceptions:

• You must use 2 hands. This doesn’t affect your Strength bonus to damage, because the rule about adding 1½ Str to damage is for “one-handed weapons” (used in both hands) and “two-handed weapons” (also, of course, used in both hands), and “one-handed weapon” and “two-handed weapon” are designations for “melee weapons.” (Personally, as GM, I would allow two-handed throwing in sensible situations to get 1½ Str to damage but that’s not in the rules.)

• You take a −2 penalty for the size mismatch.

• The weapon’s base damage is 2d6 instead of 1d8.

• It takes a full-round action.¹

1. _{See Ifusaso’s answer for more info on the rule that says throwing a weapon two-handed takes a full-round action for no good reason. I recommend houseruling that nonsense out, though.}