The dimensions of a robot vacuum matter for a couple of reasons. First, they will determine how well it can get into tight spots, such as under your kitchen cabinets and low-clearance furniture (couches and recliners). If it’s too tall, it won’t be able reach into these spots, or worse, it will get in and get stuck until you physically free it. Second, the bigger the robot vacuum, the larger the dustbin. Robot vacuums don’t use expandable bags like many of their stand-up brethren do, so when it comes to debris capacity, what you see is what you get. There is no sweet spot for robot vacuum dimensions that we could determine—it really depends on your particular room layout—but a diameter of 13 to 14 inches and a height of 3.5 to 4 inches are the most common measurements we encountered.