President Obama plans to propose new limits Thursday on the size and investments of large banks, a senior administration official said, as the White House intensifies its push to reframe its financial reform agenda as an effort to rein in the companies widely blamed for causing the economic crisis. Although the details of the new proposal could not be learned, the president plans to announce a series of measures aimed at limiting the risks that large banks can take, according to the official, who spoke before the formal announcement on condition of anonymity.