Writing short can definitely help your writing, but so can writing long. If you are in a habit of writing the story to the end, then editing it several times, etc. You can even go through the submission process. If you want to use your experience with shorts as a doorway to novel writing, then try to mimic the behavior of writing novels. If you are nitpicking every sentence in a flash piece as you write it, that might not be the right skill, unless you have a 2 year attention span while drafting a novel.
Writing flash (using the same method) is a way of gaining editing skills quickly. Scratch out a first draft of any story, then cut half the words from it. You may end up thumbing a thesaurus the whole time, but it will teach you to tighten up your sentences, and that skill will be passed on to your drafts. Your vocabulary will get better, and you will be able to find those space saving verbs quicker while drafting your novel. You will also be able to pack in more side plots and twists.
When you finish the tightening exercise, go through and double the length by adding sub plots. Make your short story the original draft length by adding more description and feeling, and weaving another thread into it. Doing this exercise will teach you to make your writing more dense, and you will start to learn how to weave, rather than just following a single plot line.
Short stories and flash fiction are excellent for developing these skills, but don't trim the story out and make it vague while cutting. Don't fluff it up while expanding. You'll learn a balance.
All that said, if you have a solid outline, I would just mark some writing time off on your daily schedule, say an hour every day, and start drafting. Stories that are well researched and laid out ahead of time practically write themselves. Once it's completed, you can go through several editing passes the way you would with a short. Brevity just makes the process faster. You will become a better writer by finishing a novel draft. You'll become better with each editing pass. And you will see your progress every time you turn back to page one.
