variable crop option with add_image_size()

OK I have found out a way to do this. With the plugin, with some checkboxes I am setting a crop option in the db in wp_options table, just like WP does it for the thumbnail, medium and large sizes.

So my big image sets the add_option('big_crop', true)

Than in functions.php I do:

if ( get_option( "big_crop" ) == true ) {
    add_image_size( 'big', 1500, 1000, true );  
} else {
    add_image_size( 'big', 1500, 1000, false ); 
}