Can I use a Large light weapon to get Power Attack damage?

Question

I'm a fighter who's found a really good dagger I want to use. Sadly, it's a light weapon and Power Attack has this unfortunate clause:

You can’t add the bonus from Power Attack to the damage dealt with a light weapon (except with unarmed strikes or natural weapon attacks), even though the penalty on attack rolls still applies.

So my damage tanks immediately. However, there is a clause in the weapon size rules:

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.

If my fighter was a medium-sized creature, and the dagger he wanted to wield was Large, he would wield it as though it were a one-handed weapon. But does this allow him to get around the Power Attack restriction and benefit from the damage?

I am not interested in house rules or DM rulings, but rules support only.

Answer 1

Yes.

Because weapons are categorized by their relative size to the wielder, your Medium fighter would treat a Large dagger as a one-handed weapon according to the following rules.

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.

This paragraph, combined with the one you quoted concerning relative weapon sizes would make your Large dagger (actually a Small Object) a weapon that your Medium fighter would be able to wield as a one-handed weapon. However, there is another paragraph in between those two that states

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.

Since daggers are simple weapons (with which your character is proficient), you would only take the -2 size penalty to your attack rolls, followed by any penalty you chose to accept from the use of Power Attack.