My question is simple: If, let's say, a Medium character wields a Small Rapier in his off-hand, does it count as a Light Weapon? After all, it should be easier to wield, or is it not because it is made for Small characters, and not Medium?
I'm playing the Pathfinder RPG system. We are using all the books, except Technology guide.
Yes, weapons come in three size categories:
There is a distinction between the object size and the weapon category size. The object size would be like the creature size but for that object when compared to another object, exactly like how creatures are compared. While the weapon category size is for what size of creature the weapon was designed to be wielded.
Which means that a medium-sized longsword is actually a small object, while a dagger is a tiny object and a greatsword is a medium object. But whenever the weapon size category changes, so changes the handedness required to wield it. A small-sized longsword becomes a tiny object, and thus a light weapon. While a small-sized greatsword becomes a small object, and thus a one-handed weapon.
From Weapon Sizes:
A weapon’s size category isn’t the same as its size as an object. Instead, a weapon’s size category is keyed to the size of the intended wielder. In general, a light weapon is an object two size categories smaller than the wielder, a one-handed weapon is an object one size category smaller than the wielder, and a two-handed weapon is an object of the same size category as the wielder.
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.
However, since the weapon was originally designed for Small sized creatures, any Medium sized creatures wielding it suffers a -2 penalty on attack rolls.
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.
Also keep in mind that this doesn't change the weapon actual size, a medium-sized longsword being wielded on two-hands by a small creature is still a medium weapon that will be considered as a two-handed weapon when being wielded like one. A small rapier is still a small one-handed weapon. This will have different behaviors when considering feats like Weapon Finesse, which requires the weapon to be for the appropriate size for your character, or Two-Weapon Fighting, which doesn't have such a restriction and will allow you to apply its effects on any light weapon.
A typical creature can use as a light weapon any typical one-handed weapon that's designed for a creature 1 size category littler than the new wielder. Such a weapon, however, is inappropriately sized, and the creature suffers a −2 penalty on attack rolls with that weapon because the weapon's designed for a creature 1 size category littler than the wielder. This penalty on attack rolls is cumulative if a weapon is even bigger or littler, but a creature typically can't wield a weapon if the weapon at all if the weapon would be less than light or more than 2-handed.1
To summarize, these are a Medium creature's options:
Light weapons: A light weapon wielded in two hands grants no benefit to damage. Weapons littler than this normally can't be wielded at all.
1-handed weapons: Wielding a 1-handed weapon 2-handed can grant a benefit to damage.
2-handed weapons: Wielding a 2-handed weapon can grant a benefit to damage. Weapons bigger than this normally can't be wielded at all (but exceptions abound).
For example, a normal rapier designed for a Small creature is usually a light weapon for the typical Medium creature. (I hedge here alot because of the vast number of exceptions available to Pathfinder creatures!) Also, while the example here is for a Medium creature, it can be scaled up or down for bigger and littler creatures.
However, using an inappropriately little—or big!—weapon can have an additional impact beyond a penalty on attack rolls (and, obviously, dealing more—or less—damage). For example, the feats Slashing Grace and Weapon Finesse and the magic item effortless lace mandate that to realize their benefits with a weapon, that weapon must be designed for a wielder of the wielder's size category. For example, a Medium creature that wields a rapier designed for a Small creature just can't benefit from the feat Weapon Finesse with that inappropriately little rapier.
As a further example, a creature can benefit from the feats Improved Critical (rapier), Weapon Focus (rapier), and Weapon Specialization (rapier) either when using a rapier designed for a creature 1 size category littler than the wielder as a light weapon or when using a rapier designed for a creature 1 size category bigger than the wielder as a 2-handed weapon, but a creature can't apply the benefit of the feat Slashing Grace nor the feat Weapon Finesse to either weapon! While a rapier is still a rapier no matter the size of the creature for which the rapier's designed, an inappropriately-sized weapon is also still inappropriately-sized, no matter the actual weapon!
On the other hand, a typical dwarf barbarian can take a fallen hill giant rogue's +1 sap that's designed for a Large creature and wield that +1 sap as a 1-handed weapon or even in two hands to realize a greater benefit from the feat Power Attack, albeit, in either case, suffering with that +1 sap a −2 penalty on attack rolls. Likewise, for example, such a dwarf barbarian could even swipe the cloud giant's assassin's dagger that's designed for a Huge creature and wield that weapon as a 2-handed weapon (only!—it's dagger is that big!) and will realize a greater benefit from the feat Power Attack, although that dwarf suffers a a substantial −4 penalty on attack rolls with such a dagger even before employing the feat.
1 A weapon's actual, for-reals size—rather than the size of wielders for which it was designed—is, typically, for a light weapon 2 size categories less than the size category of the creature for which the weapon is designed, for a 1-handed weapon 1 size category, and for a 2-handed weapon the same size category. For example, a rapier designed for a Medium creature is typically a Small object, and a rapier designed for a Small creature is typically a Tiny object.
