What to write?

One of the hardest things to decide on is what to write. Should it be a play, a short story, a novel, how many characters, a children's book,an adult novel. The possibilities are endless. So,the hardest part is narrowing down the choices.

So far I have written short subject and one act plays, a children's book, and am currently working on a mystery novel. I am enjoying all of them. So the question I pose to myself and to other writers, how do you decide?