Ok, so we started with the initial version of "The Art Of Unit Testing" and we do unit tests for classes where every test covers one aspect of one method.
This has the drawback of high maintenance costs and brittle tests and seems that the second version of "The Art Of Unit Testing" points to take bigger units when testing.
Which is the right size (the one that you use) when doing unit tests? I've heard of testing only the public interface provided by a module but in the project we are working we have a relative small public surface with a very big back office so I'm not sure if that would work as the Arrange of the tests would be really big and probably difficult to follow.
So, again, how to you set the boundaries of your SUT or how do you decide what forms your SUT?