NEW YORK/WASHINGTON (Reuters) – The U.S. Federal Reserve is in no rush to decide the appropriate size of its balance sheet, but if it ultimately shrinks it to a pre-crisis size, the process could take the better part of a decade, Fed Chair Janet Yellen said on Thursday. Yellen, in testimony to a Senate panel, said no decision had yet been made on the central bank’s portfolio of assets, which has swollen to $4.5 trillion from about $800 billion in 2007.