An ECG said that my heart is slightly enlarged. Should I be concerned?

Yes. The ECG is not always completely accurate in this regard. An echocardiogram is a more reliable test for cardiac enlargement. Uncontrolled high blood pressure is the most common cause of cardiac enlargement; control of BP will often reverse the enlargement. Disease of the heart valves or the heart muscle itself can cause heart enlargement. Ask your doctor if he thinks you should have an echo.