Story length, as Ariel says, isn't a factor under Medium guidelines. But some nommers are adding their own rules: e.g., they won't even open stories under 6 minutes (as one nommer spelled out in a post), and those rules may not be listed in the submission guidelines but listed in an archived post that gets buried in a place where you don't see it.

I've taken this up with Support and the answer essentially was: Each pub makes its own rules, and if you don't like them, you can "engage with" another publication. But if you write in a relatively narrow field that interests fewer nommers than, say, self-help or tech, there may be few or no other pubs as well suited to what you write. So you're just stuck.