Even for English speakers, the lines between novel, novella, and short story are not clear-cut. Wikipedia has 17,500 to 40,000 words as a novella, and 7,500–17,500 as a novelette; anything shorter is a short story. (It may be a generational thing, but novelette is less familiar to me.)
In Japanese dictionaries, I see both 短編小説 and 中編小説 for novella. At first glance, 中編小説 would be the better choice; but is this word used in everyday speech? And indeed, for short story, is 短編小説 more common than ショートストーリー?.
Also, I'm not familiar enough with Japanese literature and culture to know how those traditions affect terminology for historically Western texts. Are 一夕話 and コント used also for short story?