Executives at Goldman Sachs and JPMorgan Chase expressed misgivings on Thursday about the Obama administration’s new proposals to restrict the size and risk-taking of the country’s largest financial institutions. Their comments, before the Senate Banking Committee, appeared to further complicate the challenge facing the panel’s chairman, Christopher J. Dodd, Democrat of Connecticut. For months, Mr. Dodd has been leading closed-door negotiations over a bill to overhaul the nation’s financial regulations, and on Thursday he expressed dismay at how long the process was taking.