We can't know for sure, but I'm re-watching the series and I noted some stuff.
In season 3 episode 10 titled "Lois' Makeover", Hal is more interested when she has a full cake of makeup on her face
In season 4 episode 19 titled "Future Malcolm" Hal is also interested in Fat Women. He tried to make her fat, but at the same time it also seems he wants to create the perfect circular butt.
All we know is he is referencing a pre-2000 movie.
You can pick which ever suits you, the on requirement we know is that he has to go to the store.
http://www.filmsite.org/sexinfilms-index.html