Complex sentence: The museum, if built, will be larger than the San Francisco Museum of Modern Art. But the plan has engendered widespread skepticism and even outright antagonism among some historic preservationists in San Francisco.
Split into simpler sentences: The museum, if it had been built, would have been larger than the San Francisco Museum of Modern Art (SFMOMA). However, the plan engendered widespread skepticism and even outright antagonism among some historic preservationists in San Francisco.