In 2007, the Fed’s balance sheet was less than $1 trillion. Today, it is nearly $4.5 trillion. The U.S. experience is far from unique. Since 2007, global central bank balance sheets have nearly tripled to more than $22 trillion as of mid-2014. And, the increase is split evenly between advanced and emerging market economies (EMEs).

So what’s the right size? The answer depends on the policy goals and the nature of the financial system. In the case of the Fed, we expect that it will be able to achieve its long-term objectives with fewer than half of its current assets...