In my experience, the story decides how long it will be. I like mine to be lean and mean, so I do not write anything unless it has a definite purpose. However, during the editing process, I strive to shorten further, concentrating on economy of words. This basically means that anything unnecessary will be removed, and what remains will be edited for smoothness and clarity.
Over the years, I have developed a decent sense concerning the length that a work is going to be. A story that is driven by concept will tend to be shorter. The more characters that are introduced, the longer a tale tends to be... especially if there is more plot and character development.
Longer works require an expanded vision: everything is bigger and delves deeper. The characters become people that you know, and thus, it becomes more difficult to expend them. What you might think will make for a dramatic ending, can provoke a backlash from the readership. It is important to tread lightly in these matters.
I like my characters, and am loath to destroy them. There comes a time when a character will die, but it cannot be arbitrary. It must be a steady progression that leads to their demise, and usually they have paved the way to this destination through their own actions or inaction. In all stories, you set the beginning and they determine the ending.
So, a novel is not just a novel concept. It is a large work, with a lot of characters and a long journey to get to the end. In a shorter work, everything is concentrated. You avoid explanation, dropping the reader into the moment. Just like dynamite, it comes in a small package.