Do beta blockers reduce heart size? How can an enlarged heart be reduced?

They can. Over a long period of time, beta blockers and other anti hypertensives can reduce lvh.
Yes. Traditionally medications that reduce blood pressure or cause after load reduction will result over time in reduction of left ventricular hypertrophy. In patients with heart failure, i prefer to use ace inhibitors as my first line agent.
Possibly, by cause. Whether treatment reduces heart size (by echo is best) depends on cause of enlarged heart. Beta-blockers are great for heart failure and hypertension, but need to be used judiciously by skilled physicians caring for patients with the condition causing enlarged heart. Careful monitoring absolutely required due to possible adverse effects.
Enlarge heart. No, beta blockers cannot reduce heart size but can reduce fatal arrythmias.
Better by Beta. Essentially, beta blockers prevent further enlargement of the heart muscle due to hypertrophy (increase muscle fibers), due to strenuous pumping against resistance as seen in high blood pressure. Unfortunately, once the heart muscle is too large its function actually declines and the pumping action weakens. The best option is to prevent enlargement as it's not reversible once it has occurred.