While pulling off the size and height difference with the type of dress he typically wore and standing on something would pull it off in that scene - it seems highly unlikely she'd have managed to fool people between killing him and then the moment of pretending to be him in the hall. But I can kind of move past that as it is sort of what those faceless people do.
I'm not sure the size issue is even an 'issue' at all - there is something 'magical' about the assassin thing, they're not just gluing on a bit of flesh - see the scene where the man pulls off multiple faces turning into multiple people... it is more like they can turn into a physical representation of that person... the height thing is a red herring really



Great writing.