Calvin and Hobbes by Bill Watterson
Deniz sent this comic a few days ago. I forwarded it to some friends. Megan e-mailed back this response: "Being a slug on the usual scale, capable of navigating, climbing up, and squirming through a miniature natural neighborhood, would be much more enjoyable than being a monstrous slug on the scale of a city block!" I am not sure if I agree with her. There could be some satisfaction to be had from coating an entire city with slime.
But the real question is "Can a snail or slug grow that big?" And the answer is no. First, I can't think of an adaptive reason for the evolution of a monstrous gastropod from much smaller ancestors. Second, a 500-story snail would need to consume a heck of a lot of calcium carbonate to build itself a decent shell. Third, whenever and wherever such a humongous gastropod crawled, it would lose, in the form of mucus, a heck of a lot of water. Now, imagine a population of such behemoths. They would scrape down hills of limestone, drink up lakes full of water and eat up an entire tropical forest in one afternoon. The next day, they would run out of resources and be extinct by the evening.