Sometimes I want a story to be a specific length, then sometimes I find out that length is wrong for that story. I've begun to learn that to keep a story short, it often helps to have fewer focal characters, less settings, and a premise that doesn't necessarily have complex, world or universe changing consequences. And a relatively simple character arc. Small works better for short, for me. If I just write based on an idea I want to work through, then I get a pretty good idea after a while whether the story "wants" to be long or short. Lately, I've been working on a story that keeps trying to become a novel. It's been hard to pin it down to a point where I can keep it under 8000 words, and the feedback from at least half those who've read it is consistently: this could be explored in a novel; this would make a great movie; this needs to be longer... Trouble is, I'm not ready to write a novel. But I think I've done it, by working on the character arc, and getting it to a point where it can stop at 8000 words. So Dee Pratt, I do worry about length, and sometimes have to alter ideas to keep them shorter, but that's only because I don't think I've hit the novel idea that I want to write yet. Short gives me the chance to explore, and if I were to do a novel, I want to be passionate about the world I'm writing about, and not tire of it.
There are also stories I've kept to 3000 words, by vicious editing, but were much better after I expanded them again. I do think the idea has to be right for the length, and that's something learned with experience.