Many developers nowadays, including the market leaders, are using vector graphics, usually SVG files to get the best scaling effect. But in your case it will be not possible, because pixel art style does not match with vector graphics.
So it lets to you just two possibilities: create several versions of your graphics, increasing your development costs, or focus in just one resolution and accept the low quality of scaling.
Maybe focus in the low standard (480x320) can be a good idea, because pixel art style does not looks so bad scaling up.
But I'm sorry to say that this layout will be too small for small devices. You should seriously think about make the game fit almost the entire screen and overlap the touch controls.
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