Holy shit big
Refers to any object (real or otherwise) that is in fact so big, it is not simply "very big", "huge", or "gargantuan", but incomprehensibly big.
"You should have seen the size of the rats at the airport in Mumbai"
"Were they big?"
"No, they were holy shit big rats, bro"
"Were they big?"
"No, they were holy shit big rats, bro"