What is the highest sample size administered on October 18?
CREATE TABLE "polling" (
  "poll_source" text,
  "date_s_administered" text,
  "sample_size" real,
  "margin_of_error" text,
  "mel_martinez_r" text,
  "betty_castor_d" text
);
SELECT MAX("sample_size") FROM "polling" WHERE "date_s_administered"='october 18';