If you read Stephen King's "A Memoir of the Craft" you will find that he never knows how long a story is, until it is done. If you read Orson Scott Card's equivalent, he can tell you fairly well the length before he starts because he has summarized, outlined, and planed it all out well before writing the first word.
I find myself to write more like Mr. King then Mr. Card. So I sit down and write till the story is told, generally that is in the 80-90k word range. I find that is the easiest length for me to hit.
So I think its more a matter of style, then difficulty.