What is the highest value of area when capital is San Juan?
CREATE TABLE "table1_298550_1" (
  "country_with_flag" text,
  "area_km" real,
  "population_1_july_2005_est" real,
  "population_density_per_km" text,
  "capital" text
);
SELECT MAX("area_km") FROM "table1_298550_1" WHERE "capital"='San Juan';