Depending on what voice you heed, the novella is either a long short story or a short novel. Opinions vary.
Unlike the short story, however, the novella will take you more than a sitting to get through (unless you’re a fast, severely caffeinated, reader). I guess that is one of the certainties about it.
My view is that a story is as long—and has as many voices, scenes, events and other fictional elements—as it is compellingly told.
I wish you happy—and I hope, rewarding—reading.
Ulf Wolf