The Rambam writes (MT Chametz u Matza 1:1)
Anyone who intentionally eats an olive's size of
chametz on Pesach [...] is liable for kareit [...] [The above
applies] equally to one who eats chametz and one who converts it into
a liquid and drinks it.
So the question becomes how much whiskey contains an olive's worth of barley. According to this site, it takes roughly one kilo of barley to produce one bottle of 70cl of whisky. If you believe an olive is 27cm3 (Wikipedia), which for barley converts into 16.2 grams (according to this site), then an olive's worth of barley is 1.13cl of whiskey (16.2 grams * 70 cl of whiskey / 1000 grams of barley).
As to your second question, of why there is a prohibition on liquid chametz, the Gemara in Chulin 120a says
Reish Lakish said: One is liable even for drinking leavened bread. The
verse states: “For all who eat leavened bread from the first day until
the seventh day, that soul shall be cut off from Israel” (Exodus
12:15). The term “soul” is interpreted homiletically to include in the
prohibition one who drinks leavened bread.
Now all is to be chayav m’doaraisa. As is well known, one if forbidden to consume chametz on Pesach in minute quantities, as the Radbaz (quoted at the end here) nicely explains
Therefore, I rely upon that which the Rabbis taught that chametz
symbolizes the “evil inclination,” the “se’or she-be-issa.” Therefore,
a person should utterly banish it from his midst and search for it in
all of the inner chambers of his consciousness, as even the smallest
amount is not nullified. And this is true and correct.