I mentioned this briefly in another of the answers I queued up to post today, but I’ll reiterate it here: there is no limit for fic. You’re free to write as much as you’d like, though you should do your best to get as close to 1,000 words at the least.
As for the second part of your question, I’m not quite certain I’m understanding, but I’ll do my best. Each person participating turns in a survey with three prompts, and on April 1st, you’ll be sent a message with someone else’s prompts. It’s then your job to take one of those prompts (though mixing of prompts is allowed) and either write fic for it, or draw/create something for it. Each person is only responsible for one thing. But since each person will be writing/creating for someone else, everyone who gives something will also get something.
What you give depends on what you sign up for. If you sign up as a writer AND an artist, then it all depends on what your partner requests what you do. If they want fic, then you write. If they want art, then you draw/create something. If they don’t care, then it’s entirely up to you to pick which you’d rather do. You don’t do both.
I hope that answers your question! If not, feel free to send another message and we’ll talk it out in private.