Once you’ve given some thought to how detailed you want your pixel art to be, you’ll need to decide on the aspect ratio.
There’s no “correct” aspect ratio for pixel art but there are some things to keep in mind:
What are you making?
If you’re going to be doing a landscape or environment, then a horizontal aspect ratio might be best. If you’re going to be doing a character design or portrait, perhaps consider a vertical aspect ratio.
Where will this artwork live?
If it’s going to be a phone or desktop background, then you’ll probably need 16:9 (horizontal) or 9:16 (vertical). If you’re thinking about posting it to Instagram or Dribbble, you’ll probably want choose a 4:3 aspect ratio or something close to that. Will you be framing it and putting it on the wall? If so you'll need to pick an aspect ratio friendly to printing and framing. If you want to use it for multiple purposes, you’ll have to choose an orientation that’s flexible and allows you to adjust the artwork for each application.
Do you want to match the aspect ratio to a particular console or game?
Most 8/16 bit games are 4:3 or close to it. Some of the earlier ones are more square. Just do a little research and find some unaltered screenshots of the particular game(s) you’re interested in and see what the pixel dimensions are (google image search works well for this). Not every game for each console has the same screen resolution — some SNES games are much more pixel dense than others — so just find the game/style that works for you.