Your sprite can be any size you want! The size you choose depends on what art style you are going for (i.e. pixel art, realism, cartoonish).
If you do go for pixel art in your game, it's better to keep the size of the pixels in all your sprites the same. What I mean by this is stretch all your sprites the same amount so that the pixels in one sprite aren't bigger than the pixels in another. You don't have to do this but it just makes the game look better and more consistent.
