Although rats often seem huge in retrospect, when viewed at close range, it's fairly apparent that most weigh only a pound or so and are under a foot long. Of course, that isn't to say that they can't get pretty huge: this Chinese specimen, for example, weighed six pounds and had a 12-inch tail. For that matter, other rodents like the nutria or capybara often grow to a pretty impressive size.
Ultimately, size is less important than number. For most people, even one rat is far too many, and a huge colony of marauding rodents is sure to be horrifying. In this context, one resident's assertion that the rats were "big enough to put saddles on" says more about his terror than it does about the glandular monsters stalking the streets of Brighton.
That having been said, the problem is no laughing matter. Brighton, like much of Cambridge, has old housing stock and a lot of very wealthy people. Given that the town is a very fertile ground for both rats and lawsuits, it would be wise for Harvard to get on top of the problem now. After all, while there is much to be said for super-strong trash cans, they pale beside the joy of a rat-free neighborhood!