Sexual activity is generally categorized under "vigorous physical activity." Patients with known heart conditions need to consult with their physician before taking vasodilators (Vigara) or engaging in vigorous physical activity.
The specific answer to your question depends on what you mean by an "enlarged heart." Your cardiac ejection fraction, the function of your heart valves, and status of your coronary arteries (the blood vessels that supply the heart) would determine whether you could engage in vigorous physical activity of any kind, including sex.
The real answer will be obtained by a frank conversation with your physician. Hopefully knowing that this is a common question asked of doctors will make it easier for you to talk to your physician. Even if you are not ready for that activity yet, you could start a conditioning program, recommended by your physician, that could lead to that ultimate goal.