This is bigger than 8*8, yet still retro, is it not?
Retro, au contraire to popular belief, does not equal low-res. It is a style of using that low-res to make nice looking things. Now-a-days, when we see “retro” games, they usually aren’t, simply because they use a much more modernized way (and much more efficient way) of using pixel art. Making pixel-art these days takes quite some skill to replicate that of days now gone.
They would pack as much detail into the tiny spaces that they had available to them. And this ‘space’ meant more than the size they had to conform to. Artists of the “retro” era were also limited in their colours. So if you really want to retro-ize your game’s art style, I would start by increasing your texture size, and limiting yourself a small amount of colours for your entire game.
If you want some tips on using a colour palette, I would look at this link.