The length will unfortunately not effect accuracy (well it might make it easier to sight down, however after 16 inches length is a bad thing)
The design can change a few things, For example more porting (holes) the quieter but less efficent your marker. the less, the more effeicient and loud. If the porting is spiralled some people think this can have a positive effect on accuracy by making the ball a spin (like a bullet) However data doesn't prove this... Straight porting obviously doesn't make the ball spin...