Just noticed a contradiction in the rules.

Reduce Person states:

Melee and projectile weapons deal less damage. Other magical properties are not affected by this spell. Any reduced item that leaves the reduced creature's possession (including a projectile or thrown weapon) instantly returns to its normal size. This means that thrown weapons deal their normal damage (projectiles deal damage based on the size of the weapon that fired them).

While Enlarge Person states:

All equipment worn or carried by a creature is similarly enlarged by the spell. Melee weapons affected by this spell deal more damage (see Table: Tiny and Large Weapon Damage). Other magical properties are not affected by this spell. Any enlarged item that leaves an enlarged creature's possession (including a projectile or thrown weapon) instantly returns to its normal size. This means that thrown and projectile weapons deal their normal damage. Magical properties of enlarged items are not increased by this spell.

See the contradiction - two spells that are clearly related to each other (since one counters the other) yet they imply very different methods for calculating the damage dice of projectiles.

Per the Reduce Person spell you use the damage dice of the item that fires the projectile (i.e. the sling, longbow, shortbow, crossbow or firearm) even though the projectile returns to a larger size.

But per the Enlarge Person spell you use the Projectile size on impact to determine the dice - so even though the item that fired the projectile was a size larger since the projectile returned to normal size before impact you use that size to determine damage dice.

Seems logically that one of these methods is in error - and I don't know which. If the Reduce Person rules are used for Enlarge Person then that spell becomes far more powerful for archers (unclear if it would also stack with Gravity Bow since there is specific language in the spell saying that multiple size modifiers do not stack) while Reduce Person becomes somewhat of a tradeoff for ranged (projectile) attackers - better chances to hit traded off for reduced damage (while enlarge person becomes a slightly lower chance to hit for improved damage - since ranged attackers have far more ways to improved to-hit than they have to improved damage probably a tradeoff many would accept - especially with the new "adaptable" magical property for composite bows to accomodate increases in STR.

What rule is being contradicted?

Reduce person states that projectiles deal damage based on the size of the item that FIRED them.

Enlarge person states that projectiles deal damage based on THEIR size (not the size of the item that fired them).

Seems to me, at least, that both rules can't be right since they are contradictory ways of arriving at the damage dice in a size-change situation.

(and unclear if there is a third, generic rule somewhere to handle all the many other ways in which size might change)

It's just fine. The smaller weapon can't impart the same energy to the projectile (even though it returns to normal size when fired) than the normal version. The larger version can impart more energy, but the projectile can't retain it when it returns to normal size. Magic really messes with the law of conservation of energy/mass, but then again that's what magic's for.

EDIT: @Rycaut: Projectiles do damage based on the smaller of the launcher that fired them or the size of the projectile.

Think of it the way that the strength of a chain is determined by the weakest link.

The law of balance overrides the law of internal consistency in all things. mmmm

Shrug. I rule that the projectiles strike in their modified form and then return to normal size.

Conceptually, I want both magical effects to work in similar ways. I try to preserve consistency when possible.

Mechanically, I don't like the idea that an Enlarged creature is receiving an attack penalty, and receiving no benefit; but a Reduced creature gets an attack bonus without balancing factor on damage.

But then, I have more players that want to run around as perpetually-housecat sized +8 Racial Stealth Sneak Attack munchkins than I have players running around wanting to be Large or larger and hurling things, so my balance issues may be of a different shade than other GMs'.

Shrug. I rule that the projectiles strike in their modified form and then return to normal size.

So reduced arrows strike as their smaller form and then get bigger in the wound? :)