A man leaves a 7-Eleven store with a Double Gulp drink in New York. If New York City bans big sodas, what's next? Large slices of pizza? Double-scoop ice cream cones? Tubs of movie-theater popcorn? The 16-ounce strip steak? Opponents of the proposed ban may use that slippery-slope argument along with other legal strategies to try to block the first-in-the-nation rule.