Most of the time, the retro look comes from the game using a Virtual Resolution, or Nearest-Neighbour scaling (which preserves pixelation if you're scaling to resolutions that are multiples of the original - else it doesn't map 1:1 and the result is distorted).
A virtual resolution is used to represent graphics with an appearance differently than that of the graphics context of the application - in other words, you can use it to display pixels that look bigger.
If you want your game to have a 480 x 800 resolution and still look retro\pixel-art, you can use a virtual resolution of 240 x 400. This maps in a way that the pixels will look two times bigger than they normally should.
This means you can open your painting program, create a canvas of 240 x 400 and start drawing your graphics with pixel art. When you're going to display your graphics in your game, they'll be scaled 200% to perfectly fit the 480 x 800 area.
Always do it like that, by having the engine scale the small graphics up to a bigger size.
Never draw the graphics directly with the doubled size, otherwise you would be wasting memory.