Below are definitions of what a short story, a novella, a novel, and an epic are! 

Short Stories;
• Under 500 words can be described as 'Flash Fiction'
• Between 1000 and 8000 words is a short story (also, most short story competitions will stipulate their required word length for entry)
• Between 5000 and 10,000 words is a long short story

Novella;
• This is a story that is between 10,000 and 40,000 words.

Novel;
• A manuscript over 40,000 words is considered to be a novel. However, very few novels these days are as short as that. Generally, a 50,000-word novel would be the minimum word count. Most novels are between 60,000 and 100,000 words. 

Epics;
• A single novel can be longer, but once the length is above 110,000 words, publishers may look at cutting it back unless it is a particular kind of book - books over the 110K word count are usually considered 'epics'.