There used to be a time when a sprite had to be a certain size but those days are over. Any multiple of 8 will usually look good so 8x8 16x16, 32x32 etc.
I use 32x32 for most of my games.
What does matter is how big the image is that you are loading into a sprite. Very large images will lose pixels and quality when reduced and vice versa so I try to stick with images around the same size as the sprite they will be used for.