Easily solved, @Anna140. Open your calendar. Look for "Availability Settings" near the right side of the dark gray bar near the top of the page. Click that. Near the bottom you'll see fields for Minimum and Maximum stays. You should be aware of your local laws regarding tenancy rights before you set a long "Max stay". Some localities give full tenant's rights to visitors who stay more than X days, where X might range from 28-32 nights, etc.
Note that you beside setting your default minimum, you can use the link just below those fields to set different minimums for specific dates. So, for example, even though you say you prefer 7-night rentals, if you ended up with a 5 or 6 night gap between two 7-night stays, you could make that gap available for searchers if you wanted to.