I agree that short pieces thrive here, but FYI that longer pieces can find an audience too. I have two 20-minute pieces on Medium, and both have been curated into three topics:
Abortion Stories: Three Women and a Clinic (a literary journalism piece) curated in Women, Family and Equality
Sixty-Seven Hours of Homebirth (my personal birthing story) curated in Women, Family and Parenting.
The longest piece I’ve personally seen is the Medium-featured The Good Guy (fiction by the author of Cat Person), at 62 minutes. I started reading it and bookmarked it to finish it later; I bet a lot of people did the same.