I think the most famous, and most limiting, writing assignment is the six word story. There are various anecdotes that Hemingway was challenged to write one on a bet, that he did to show off, etc. Snopes.com discusses it here. For me, I guess I don't really care if it's true or not; it makes for a good story, and I like that kind of hearsay, to be frank. But, the story alleged to be Hemingway's six word story is as follows:
For sale: baby shoes, never used.Six words, and they evoke a novel of loss and heartbreak. Could you use that as the leaping-off point to write a book? Of course you could. You could read those six words and think "what if?" and "why?" and then you're off, and you can use those six words for a title. Or, those six words tell the entirety of the story that's needed. "Less is more", and all that.
Another, increasingly more common, limit is 140 characters. When you're on Twitter, that's all you get. If you're a weirdo like me, you don't actually like abbreviating words, either. I like to write full and coherent sentences, using the proper words. Oh, I'll use a number instead of spelling it out, and "&" is my friend, but Twitter is a tool for conversational brevity that shouldn't be underestimated. A lot of people post stupid things on Twitter, to be sure. Some people post other literary works (One of my favorite Twitter feeds is @HowlTweeter, which post's Ginsberg's Howl, every hour on the hour, a line at a time.) It's lovely, sometimes, the way it fits into what others are Tweeting. Last night, a lot of people were Tweeting and Retweeting Occupy Wall Street information, lawyer phone numbers, and where the police were. It was very Orwellian.
Last but not least, is another word count limit challenge, with a time limit, one that I endeavor to do once a year (and now, with the Camps, more): National Novel Writing Month. Starting this year, in July and August, there's also Camp NaNoWriMo. The challenge is to take the month and write a book. Start the first line on the 1st, wrap it up on the 30th (or the 31st, in the case of the camps). 50,000 words in a month; that's as long as Of Mice and Men, or The Great Gatsby. You can write more than 50k words, of course, if you feel up to it, but the challenge is to create a fresh work of fiction during that time.
Quantity, not quality, is what a lot of people say about it, jokingly and not. There's a lot of "cheating" discussion on the forums there, for how to bulk up your word count. The only "cheat" that I do is use epigraphs, but I do anyway; it's something I dig. What I'm enchanted by is the pell-mell nature of the endeavor, to just get it out of your head and on the page as quickly as you can, damn the torpedoes, fix it up later! You're a writer, write! It can be exhausting, it can be madness, and it highlights the importance of writing every day. It highlights the importance of being brave enough to write.