WASHINGTON, Nov 10 (Reuters) - Large global banks continue to pose risks for policy-makers, and requiring them to be capitalized separately at home and abroad may help ease some concerns, a senior U.S. Federal Reserve official said on Tuesday.

"The largest institutions have over time become larger relative to the size of the economy," Boston Federal Reserve Bank President Eric Rosengren said in remarks prepared for delivery to a seminar in London.

"Banks that are global, not just large, can create additional complications -- they are more difficult to resolve and can 'export' capital adequacy problems ... to countries that host their operations," he said.

A text of his remarks was made available in Washington.

Rosengren's remarks come as policy makers and lawmakers debate reforms to the financial system in the wake of a crisis that destabilized economies around the world.

A consequence of worries about how bank problems can upset domestic economies may be that regulators encourage globally active banks to expand domestically but shrink internationally, Rosengren said.

Many countries may decide to insulate their banking systems by requiring domestic operations of global banks to be separately capitalized, he said.

Rosengren said he is skeptical of proposals to narrow bank activities. Instead, to reduce systemic risks, he said he favors requiring greater capital cushions and mandating institutions to set aside more reserves during boom times.

Measures should be put in place that would smooth access to capital for important firms during downturns, Rosengren said. Mandating conversion of debt and cutting dividends would help banks retain capital during downturns, he added. (Reporting by Mark Felsenthal, Editing by Chizu Nomiyama)