Among the zip code under Saint Croix county, which zip code has the biggest land area?
biggest land area refers to Max(land_area); "SAINT CROIX" is the county
SELECT T1.zip_code FROM country AS T1 INNER JOIN zip_data AS T2 ON T1.zip_code = T2.zip_code WHERE T1.county = 'SAINT CROIX' ORDER BY T2.land_area DESC LIMIT 1