Your question was how to change the "resolution" of the UI, and manual size is how it's done. It doesn't mean you have to keep it at "manual" after you test the behaviour.
To whatever resolution you are trying to test.The best thing to do would be to have a dual monitor setup and detach your game window so that it floats on the 2nd monitor. Failing that though, the fixed size is the approach to take.
Again... use FixedSize + aspect ratio setting in the game window to test how UI will look at your target resolution.When it's time to publish, change it to PixelPerfect, and hit the Build button.Do not change anything at run-time.