Like Johonny said, it depends on where your talking from. Wild rats, such as the infamous ones in New York City, are MASSIVE (think cat or dog-sized.) And they tend to cause alot of problems in the subways and the trains. But this also means that they are incredibly smart, and one of the most adaptable species of animal I know.
The only difference between the animal and the human, are the letters that they are spelled with.