Actually nothing in the rules says anything about this, while anything affecting the size of weapons seems to not stack with other kinds of growing effects. It is mentioned for the Growing weapon special ability and the Overcompensation spell that they don't stack with any other effect of the same kind, for example you cannot make a Medium weapon grow to Huge size with Enlarge Person on yourself and the Growing special ability.
Then I found multiple ways for a character to temporarily grow in size :
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Enlarge Person : 1st level arcane spell, exactly what it says on the tin, affects humanoids only ;
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Righteous Might : 5th level divine spell, comes with other benefits than just growing, affects the caster only ;
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Demonic Bulk
: a Bloodrager bloodrage power from the Abyssal bloodline, same effect as Enlarge Person but personal only and not limited by type.
In all these cases, there is no disclaimer of the same kind. Does this mean that these effects can stack on the same character ?
Best Answer
No, as both Enlarge Person and Righteous Might say:
And Demonic Bulk references Enlarge Person except the target doesn't need to be a humanoid, so that would also be covered.