The girl "Tohno Akiha" turns giant when her "loyal" maid Kohaku injects her with a drug which Kohaku calls the "Makikyuu X".
The characters are from the melty blood visual novel game.
Some of these characters were adapted into the tsukihime series.
Melty blood has been adapted into various 2d fighting games where giant Akiha is a playable character.
heres a gameplay clip:
Now I have no idea when this event occurs in the story but according to the translator notes of tsukihime doujin that was posted in the SW section recently, the maid Kohaku was possesed by a vampire Tatari whos a villian in the series.
I found some pics from a doujin that feature Giant Akiha as you can see.
Right now theres a tsukihime, melty blood and melty blood 2nd act manga.
I have read through melty blood manga so I`ll be checking out tsukihime and melty 2nd act.