Pinal,
Unfortunately, the descritive term 'enlarged heart' is not a medical term and leaves me as lost as you on exactly what condition your nan may have.
I would assume she has one of two conditions. Either ventricular hypertrophy or thickeing of the walls which is most commonly associated with a history of high blood pressure or a dilated heart associted with impairment of ventricular function. The latter is most commonly caused by hypertension also.
A start would be to find out which condition she may have and that would be done with an echocardiogram. Depending on which the medications she should take would vary but in both your would strive for good blood pressure control and good cholesterol control as well as an active healthy lifestyle. Given her chest pains, wou would want to ensure she didnt have coronary blockages causing her problems in either condition.
To start, you should figure out what your doctor is referring to by enlarged heart and ask him/her exaclty what they are doing to prevent it from worsening and where they think it came from.
hope this is a start.
good luck