(Upcoming) RetroArch 1.7.6 - Resolution selection! (Windows)
We have implemented a dropdown list of all the available resolutions that your display server currently supports. You can select a resolution from this list and it will change the output resolution, and it will even change the refresh rate accordingly. So you will be able to jump from 4K 60Hz to 1440p 120Hz if your video card and TV/monitor allows for it on the fly!
Hopefully this is far more convenient than having to manually set the fullscreen width/height manually inside Video Settings.
Note that this is currently implemented for Windows only, we will have to see how we are going to be able to extend this across other platforms. Linux is a likely next target, and PS3/Wii had some existing resolution handling that we should probably refactor to use this system instead.