One size does not fit all. We hear that all the time about different things; well, the same goes for writing. There are different lengths of writing; depending on certain features of your story, you may be writing the wrong type of story!
1,500 WORDS MAX
You can get through a flash fiction in, well, a flash! Short and sweet, you get right to the point with this. No need to get all complicated. Think of it like one scene of a bigger story.
1,500 - 7,500 WORDS MAX
Short stories are mini-novels. They do tell a full story, but not a very complicated one. Usually with one conflict that goes through the whole sequence of events, this can be similar to a chapter.
20,000 - 50,000 WORDS MAX
Novellas are one of my favorites! I love reading and writing them because you can get a full novel in half the time. Simple versions of a novel, think of it as just the meat and bones.
40,000 WORDS MINIMUM
Everyone knows about novels. The most popular form, you have much more freedom to get creative with one of these. But, don't get carried away. Because there is so much room, you can mess up your story easily!
Maybe your ideas just can't stop. You have your novel figured out, but it doesn't feel done; there's so much more to the story that you can't fit in right now.
You probably need a series. Write a novel for each one, make sure they reference one another and are cohesive, and put them all together!