A common length is about 50 to 100 pages, or around 20,000 to 40,000 words. The extra length is generally used for more character development than is possible in a short story, but without the much greater character and plot development of a novel.
"[The novella] is one of the richest and most rewarding of literary forms...it allows for more extended development of theme and character than does the short story, without making the elaborate structural demands of the full-length book. Thus it provides an intense, detailed exploration of its subject, providing to some degree both the concentrated focus of the short story and the broad scope of the novel." (Silverberg, vii)