As always, I think of the novella 'as a container, not its contents'.
Similar to Edgar Allan Poe, I like best a text (cum book) that you can read in one sitting and which results in one overall impression upon the reader.
So I have stolen the term 'novella' from the literary types, who use it most consistently for fictional work of between roughly 12,500 words and 25,000 words.
In practice, a novella today is usually around 19,000 words long.
(I think most of us regard fiction between 7500 words to about 15,000 words as either a novelette or a long short story, just as we tend to regard fiction over 30,000 words in length as a short novel.)
In terms of work that isn't fully or overtly fictional, like my work, this results a book of between 50 and 120 pages, depending on illustrations and formatting.
I think most of us can read a book like this, if it is clearly written and attractively laid out, in between 75 and 150 minutes - the same range of length as most movies/plays/ musicals and concerts.
My 'book' , "MO goes PO, 1939-1945" , is actually a series of 16 separate but connected essays easy of novella length.
Hopefully each will average around 12,500 words, giving an overall length for the entire series of 200,000 words.
I have had to condense the story greatly.
This meant I had to pick just sixteen key days, over a war that lasted six years and raged over 2000 separate days.
As a result, I have gone back 'n' forth/ forth 'n' back over the specific sixteen dates, so I can still tell the story succinctly but accurately and dramatically.
So here are my tentative dates and titles - always subject to further change, of course:
January 1st 1941 : "We Are The Dead".
March 15th 1941 : "Synthetic Hope".
May 20th 1941: "Life Unworthy" .
September 30th 1941 : "Thank You - And GOODBYE" .
January 30th 1942 : "Kristall Lust" .
March 12th 1942 : "Patient Betrayal" .
June 30th 1942 : "Censored" .
November 6th 1942 : "Going Native" .
January 29th 1943 : "Hill Of Beans" .
June 14th 1943 : "When Doctors Gossip" .
August 11th 1943 : "Fire This Time" .
October 30th 1943 : "Patricia, Harriet and Mae" .
January 25th 1944 : "Saved By Face" .
March 19th 1944 : "The Great White Hope" .
June 23rd 1944 : "Lazarus, Too" .
August 15th 1944 : "Blanket Harbour" .
Some will also have explanatory subtitles ...