There is no correct ratio.
you could make it 16:9 or 4:3 or 14:9 or 1:1
Actually anything will do depending on what kind of game you are making.
If your intention is to make a game that runs fullscreen, you should make sure that after scaling the bits of the screen not used for your game have some kind of graphics on them, or are a background-colour suitable as a background to your game.
If you use scale-inner nothing outside of your "playing-area" will be rendered, but on devices with resolutions very far apart from your design-choice chance is that too much of your "playing-area" is cut of.
if you use scale outer Construct will make sure your whole project-size area is on screen and make visible all the surroundings on your layout that fit inside the open window area.
these two options are best for real full screen, but you'll have to choose which one suits your need and create your layout accordingly.