In several interviews I've read or listened to from Brandon Sanderson, he says his writing strength are epic novels--not short stories. He has a new novellas out in the market, but that's about as short as he likes to get. He doesn't even try short stories because he knows he's not good at them.
Last week during our tri-weekly writers group meeting, the other member told me her strength is in writing short pieces--250-300 word shorts. This is great for her to realize, and now she's on a plan to start down the path towards collecting these pieces and self-publishing them.
Are short stories your thing?
How about 75K word novels?
Epic novels that topple 200K easily?
If you're unsure, try writing in as many veins as possible to find out where your niche lies. Maybe it's all of the above. Whatever it is, write to your strength.