Big or little?

Yesterday I got a children's story idea and started to scribble.  Soon, however, I realized that I didn't know whether I was working on a short story or a book.  Should I flesh it out into a 15,000-word chapter book or shear it down into a 1500-word tale?  This particular confusion has not happened to me before.  Then again, I've never written a chapter book, so maybe I just don't know how to recognize a potential one.  Maybe it's time to try.