Quote Originally Posted by Kitsuneymg View Post
Except this isn't a Paizo produced product. Given that every other effect in PoW designed to increase your effective size explicitly mentions size increase (as if one size larger), it's not a size increase, but rather, exactly what it says: one dice step increase.
Short simple answer: Whether it's 3PP or not is irrelevant. If the Improved Damage evo increases the effective size of the damage die per RAW, then so does DE per RAW.

Long answer: If there is a precedent set by Paizo which uses a specific wording, whatever is true for that precedent is also true in the case of any other option/rules item made for the PF system which also use that same wording, including 3PP options. Note also that in this case, an actual precedent made by Paizo isn't even necessary, as their next FAQ entry also use the exact same wording as an example in of what constitutes an effective size increase.

Of course, all DSP content adheres to the rules of the system it was written for, meaning the RAW of at the very least the PF CRB and (Paizo's) related FAQ entries apply unless otherwise specifically mentioned in the DSP content in question. And while an exception could perhaps be made if the rule in question was changed/clarified by Paizo after DSP's content was released, in this case both relevant FAQ entries were published (and thoroughly discussed) in March 2015, months before DE was even released for play testing, and a year before PoW:E was released.

In conclusion, if the intent of DE was to increase the damage die to 1d10, then the feat would have to explicitly say so. Likewise, if this damage die change were intended to stack with that of effective size increases, the feat would also have to at the very least use a different wording. (Like say: "The base damage die of a katana you wield becomes 1d10 (if Medium size, 1d8 if Small). This change stacks with that of damage die size increases.")

As is, the RAW of DE is that the damage die becomes 2d6 and that this increase does not stack with any other effective size increases. And as is, the only way around this would be to house rule the feat.

RAI? You'd have to ask Elricaltovilla who wrote the feat. (And if he says that the intent was 1d10 and he cannot see how the feat's RAW could be read in any other way, please be a nice guy and refer him to this post so that he may better understand the issue.)