I hear "short story" and "novella" referred to as genres, but it seems odd since I never hear "novel" called a genre. And then when you reach "epic," it's suddenly a genre again. But that might just be me.
I guess you could call them different classes of the same medium, but that's so... formal. Isn't there a word from some other language English can steal to make up for its own shortcomings? English is good at that.