Sumo Bots is a building event in C division in 2011. The event consists of building a robot which is able to push another team's robot out of a designated area.
There are lots of ways to build your robot, you can use vex or some other sort of kit, strap things onto an RC car, or custom build a robot from parts you attain yourself and are not part of a kit. The method that has had by far the most success is the custom built robot as it allows you to put as much motor and wheel into the maximum space and weight without being constrained by a kit or likely underpowered RC car.
Under the trial event rules Sumo Bots is a bit of a battle of the wallets as most of the components to building a good robot cost a lot. If you can salvage parts such as wheels and motors from old robots or RC cars and the like you may be able to reduce this cost a bit. Hopefully when the event becomes an event the national committee will make some changes to the maximum size and weight of the bots so as to make the event less dependent on money and more dependent on who has the best design and who is the best driver.
There are lots of different design possibilities, especially if you're custom building your Sumobot, but by far the most popular design is the wedge.
There are a lot more variations on the wedge that make different designs more or less competitive, however, beyond making the scoop lower, sharper, and more steep (all of which are obvious though important), but they'll come to you more easily by intuition and experience than by reading about it here.
Not an ideal design as even a bad wedge can probably beat it unless you are able to out maneuver the wedge and hit it on a non wedged side.
More wheels mean that less weight rests on the scoop. The scoop does not contribute to pushing power, so weight resting on it is weight wasted. Having more wheels generally means that you will have wheel slip when turning. This happens when the wheel isn't pointing in the same direction as its motion, such as in a four wheeled robot turning about its center. All wheels are pointed forward or backwards, but their direction of travel is tangent to circles centered at the turning point.Either will contribute to significant loss of pushing force; the first for obvious reasons and the second because it will cause the wheel and the ground to engage in kinetic friction instead of the static friction of a wheel rolling on the ground.
I think it goes without saying that you want the most powerful motors you can get that are legal under the rules.
More then any other event, it is key that the person operating the Sumobot has spent lots of time with the Sumobot and is a skilled operator capable of thinking on their feet so they can react to whatever their opponent throws at them. You can have a Sumo bot capable of knocking a person out of the ring and you aren't gonna win anything unless you have a driver who knows what they're doing.