I would like my Shotcut-edited video to be 1280 pixels wide and 720 pixels high when I export it. Is there a way to tell the editor to make it this size, and if so, how?
When you start your project, pick your video mode.
Save your project when done editing, then Export.
You do that right at the start of the project by setting up the project details where it says “New Project” and hitting “Video Mode” to set the resolution and fps. That’s the new UI that debuted here in the latest Shotcut version. You can also set the video mode by going to “Settings” and picking the Video mode there. If for whatever reason you want to change the resolution right before export then you choose Advanced mode in the Export menu and change the resolution in the video tab.