Draw whatever you want but it should be at least 1.5 longer in one dimension than in another. Pixel art is usually about squares, tiles which just repeat. If the sprite is 10px width and 15px height it is good. But if it is visually will be more square than rectangle so. Just do something close to 2+/1 ratio and it will be good. Swords, trees are objects which are usually looooong.