When a doctor tells a patient they have big lungs, is that a good thing or a bad thing?

Depends: Having big lugs is not necessarily dangerous. Tall people can have big lungs. It may also be a sign of hyperinflation or trapped air in the lungs. This is associated with emphysema or asthma.

It depends : It really depends on what you mean by big lungs. If you mean that your lungs are large, this can just be normal for you where is it may not be normal for someone else. If you mean that the lungs are hyperinflated, that can be a sign of Air trapping from various disease processes such as asthma, emphysema, or COPD. You should really discuss this with your doctor in light of your medical history.