score:4
Hmm; two flight search engines come to mind at once:
Google Flights: its "Flexible Dates" and "Price graph" features are both perfect for showing you the price of trips of a given length. Using Flexible Dates, you can just browse on the diagonal; Price Graph explicitly lets you specify the length of the stay.
Granted, it sounds like you're also looking for a list of flights (rather than the price); there's still one extra click before you see those for each day. However, it's probably the best tool I know for this!
Hope this helps!
Upvote:1
One more occasion to laud Kiwi (see here, here and here as well).
You can specify both a date range for the departure and a range for the time/duration of stay in nights.
Additional features include connections on multiple tickets or great flexibility in destination and departure airport. Highly recommended.
P.S: this direct link to the query pictured above should be good until Jan 2018 (when the departure will be in the past).