I use the Cargo Cashe on my utility bike and had it installed on my road bike for a while.
It works great for me since I'm tall and use a size 60cm frame. This allows me to fit the box under the seat, with a portion of the box is ahead of the rear axle. The box is less likely to fit well on a smaller bike behind a person of average height IMO. In this case, the box would need to be placed behind the seat and most of the box would extend behind the rear axle. This would limit the stability of the bike if loaded with more than 20 lbs.
What do you need to carry? On what bike are you planning on installing this item on?
Michael