I've very rarely seen US based airlines give you an issue about the actual size. The exception are on the small commuter jets or if you're boarding late in the flight and there's no bin space left.
International flights are a bit more strict on it, but then if you have a large roller bag, the weight is also an issue.
If you really want a bag that can hold a lot and actually fits the sizing requirements, get a tough soft-sided bag that you carry on your bag or shoulder. Red Oxx's Air Boss or Sky Train and Tom Bihn's Tristar/Western Flyer or Aeronaut are good options to start looking at.