I''m sure they''d love to use a 757, but they would want to test out the route first.
But, given that everybody else flies 757s and 767s to the Left coast, there should be no problem filling an A319.
LGA''s runways are 7000 feet. Given that the A319 has moderately long legs, it should be doable. But, I''ll defer to the weight/load experts.