@Allison32 To do that, go to your calendar. Near the right side of the dark gray bar near the top, click on Availability Settings. Scroll down until, near the bottom, you see "Add a requirement...". Click on that. In the box that pops up, click on the field to the right of "During" and select "Specific dates" from the list that appears. Specify your start/end dates and fill in the minimum stay for that range. Save and you're done.
Don't *forget* that you've set that custom minimum. I've seen other hosts get confused about why someone can't book a short stay, forgetting about the additional requirement they put in place months before.