One of the the sad facts I've had to deal with is that length does matter. Yes, this is a hard thing for any writer to swallow, but editors have needs, and we must rise to the occassion, as the bishop said to the actress. Their needs are often very specific. Yes, a story is as long as it is, but the antholgy isn't buying anything longer than 6000 words unless you happen to be Stephen King. And if the retailers are calling for shorter Romance or longer Fantasy, you, dear writer, had better shape up to fit the gap... or there will be no gap.
Now Sarah can probably chime in and give some more precise figures, but the odds are the length you will need to provide for the most common markets are 4000 -6000 words, or in general novel terms around 90-120K words. There's stuff like folios and print sizes and shipping boxes... all of which are not your problem but your publisher's problem. However providing a satifactory length book or story does make you a little more popular. I figure I've written about 5 million words of books and stories by now, and I have something of a feel for how long it will take me to tell a specific story. That's very nice for me... but not much use as advice, I hear you mutter, before preparing to pelt me with over-ripe fruit. Well, yes. Fortunately I have a hard head, and washable pelt, but the truth is length is rather specific to the individual. I'm quite terse, so the same story will take me two-thirds of Eric's natural length and so on. The point is you have to practice this and learn it, but there is clear relationship between the number of main characters and the length, and a less direct relationship with scenes. Each writer varies, and you will have to experiment and find your own, but for me it's 10 to 15K for each major character, and for short stories about 1.5-2K a scene (hence my shorts often tend to 3 scenes). That works for me. It won't be the same for you, but it is worth coming to grips with your length if you need to insert extra (or cut more) to be the object of your editors desire.
Here endeth the entendre!