Do you have shortness of breath when you walk? Do you have a hard time lying flat in bed at night? Any chest pains? Do you have hypertension? Cardiomegaly
is more likely to be present if you have one or more of these symptoms.
Although your best guess estimation by physical exam may be correct, I wouldn't fret. To confirm, best to get a 2 view chest x-ray, or even better get cardiac ultrasound.