Here is a link to Mayo Clinic which provides information on the potential causes of an enlarged heart:
http://www.mayoclinic.com/health/enlarged-heart/HQ00623
Did the doctor provide any additional information as to why your Dad's heart might be enlarged? Does he have shortness of breath, dizzy spells, arrythmias, etc? Is he able to exercise (per the doctor)?
Is your Dad taking any medications for his heart?