You could also use total word count as a length target. Shoot for 60K-90K total words, however many stories that takes, and you have something that is roughly the same length as a novel.
I have an anthology with 13 themed short shorts in it. It's only 11K-ish words total, but it seemed to work as a package.
I have 2 other anthologies in the works. One will be just 4 stories (3 short stories and one novelette) (about 30K-40K total words). The other will be 15-20 short stories (70K total words). Originally they were going to be a single anthology, but several of the stories became their own little subset, so I decided to separate them out.
I originally wrote
The Summoning Fire as a collection of 23 connected short stories. After editing, though, I'm not sure much of that is still visible. It's definitely a novel now. There are a few "chapters" in the book, though, that could function as standalone stories.
-David