I really don't think they're going to be that nitpicky over length. I think I've mentioned it in this thread already, but they just want a good amount of your work done before they consider publishing it - as they get a lot of people who submit like, a few sentences, or just a general idea, but don't have any actual work to show and prove that they'd be a good fit on the Tapas app (remember that the books are profitable on the app, and Tapas is a business - they can't be putting money into a product that isn't going to give them return in quality content and consistency).
So like, get the minimum done, but if you get more than what they were looking for done, more power to you. Unless they just want short stories, they're probably not gonna be like "AAAGHHH YOU WENT 30 WORDS OVER THE LIMIT, YOU'RE DISQUALIFIED". They just want to see that you're actually dedicated and have some work to show what your writing is like and if it would be good to publish on the app.