My husband was diagnosed with borderline cardiac heart size. what does this mean?

It depends: It depends on the type of test which he has had done. Most commonly such reports come from a routine chest x-ray which is not always reliable for mild enlargment. I would not worry about it unless he has any reason that could cause enlargemnent of heart such as high BP or heart murmur. If there is any ongoing concern a cardiologist can check him further with an echocardiogram which is more reli.

Heart: Every organ has physical parameters with a range of size considered normal. Borderline cardiac heart size means either at the upper or lower limits of the normal range for size of heart. This doesn't mean something is wrong it just defines where the size falls in the range. Could be normal size and abnormal function etc................