SELECT *, ST_Area(the_geom::geography,true) alan FROM pl_park

SELECT subquery.*
FROM (
  [your query]
) AS subquery
WHERE ST_Area(subquery.the_geom) > [some_value]

ALTER TABLE pl_park ADD COLUMN area numeric;
UPDATE pl_park SET area = ST_Area(pl_park.the_geom);

SELECT *, ST_Area(the_geom::geography,true) alan FROM pl_park WHERE ST_Area(the_geom::geography,true) > 1000