Changing thumbnail size

Hello! I’m trying to change the size of the featured image, but I’m not being capable. I’ve tried:

the_post_thumbnail( array(280,160) );

And included in functions.php this:

add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 280, 160, true );

But it only obbeys the height parameter. Why is that? Any idea?

Thanks a lot!