The artwork in any retro console is stored in the game as 8x8 px tiles.
To explain this, there needs to be further discussion on how retro consoles store, use, and display art. The restrictions on palette use get even tighter as the artist moves on to how the palettes are used in the game. Instead, enable cropping so these users will have margins, rather than having to play in a resolution which doesn’t fit their screen. However, keep in mind that not all users will have a display setup that will work well with your limitations, so this is not recommended. If you find a particular configuration works just as you want in the ratio you’re targeting but looks bad in some particular aspect ratios, you can prevent the window from being at those ratios here. In Unity’s toolbar, you can go under Edit > Project Settings > Player and limit the aspect ratios that the game will support. For example, 320x180 reference resolution is 16:9, and so it will have no black bar margins when played at 1920x1080 or any resolution which is an even multiple of 320x180, such as 1280x720. At the time of writing, most gamers play on 16:9 monitors, and in 1920x1080 resolution.
We recommend setting the camera to be optimized for 16:9 aspect ratio viewing, including reference resolution if possible.