My form was 2 letter-sized pages. The first page had the real info (arrival and departure in separate panels) and the 2nd page was just the instructions. Why bother printing the instructions on the back of the stuff that matters? I can bring the 2nd page with instructions.
BTW, American sent me a reminder about forms and such for the flight to CUN next week. They suggested downloading and printing both the FMM and customs forms and had the official link to both. In a typical snafu, the site for the customs form said clearly the forms were samples and not to be used, and you should use the paper card given on the airplane. I downloaded the customs forms anyway, and again it was 2 pages, but this time the real info was half on one page and half on the other, with instructions taking up the other halves. I looked at the official site someone gave above where you can enter your customs info electronically, but you can only fill it out on the day of your arrival.
As given by American Airlines:
Immigration form
Customs form