I'll say it again, they are not house rules but interpretations of a vague rule. I agree that it
wouldn't be bad if FFG released a clarification on the issue, but it's not an error in the rule,
so it doesn't require an errata.
About using the size of the standups. Is the Wargor a large standup (I don't remember
and haven't got it at hand)?
So using the size of the standups are a creature 'size' classification system isn't a house rule in your mind...
The Troll-feller Strike card reads: "Melee weapon equipped, engaged with target, you must be outnumbered or facing a physically larger opponent."
The term 'outnumbered' is not vague - it is precise.
Ref: www.-m-w.com : to exceed in number
If it had been worded 'significantly outnumbered' then this would be a calling on the GM to judge what 'significant' meant. As a GM I do apply a 'significant' qualifier to this card - but I don't pretend this isn't a house rule. However, in order to have a challenging encounter I often have the PCs 'significantly outnumbered'.
The desciption of Beastmen in ToA p50 states:
Ungor Warrior: Have the size and build of a strong, hale human.
Gor Warrior: Standing a foot or more taller than a ungor warrior.
Wargor: A massive and terrible creature.
A reasonable interpretation of 'physically larger opponent', is larger in both mass and hight. A strong dwarf, human or elf is about the same mass as an Ungor. However a Gor stands a foot+ taller i.e. 6'7" or more. Very, very few modern humans are as tall as 6'7". The number of 6'7"+ humans that are 'well build', as I imagine Gors to be, is even lower. Mass is roughly a function of hight cubed - so a Gor is a significantly physically larger opponent than any non-mutant human, elf or dwarf PC.
To only allow the card to apply to Wargors, would to be effectively rewording the card as 'a hugely physically larger opponent ' - imo that not even a modest houserule, is a bold one.
The beastmen models in the Core and the GM Toolkit are both the standard 44mm hight - one I interprete as a Ungor and the other a Gor. Imo classing a Gor as not 'significantly physically larger' than a human/elf/dwarf is absurd. The only larger standups are, Rat Orge, Giant and Troll. The only smaller ones are a Snotling (I think) and a Nurgling.