Hello,
as per description on nwn wiki and in game log, you should not be able to knockdown a character, which is two sizes difference from you.
This mean, if you ,as human (medium), want to KD a dragon (huge), game will tell you " A character can only make a knockdown attempt on opponents that are one size category larger than himself, or smaller." Which is correct and works well.
On the other hand, if you as larger character want to KD smaller character, you will be able to, but should not.
Tested on PGCC.
Human trying to KD dragon (huge) - cannot.
Human trying to KD imp (tiny) - can (ERROR!)
Ogre (large) trying to KD halfling (small) - can (ERROR!)