We machine it to a custom length. Using two of these you can make your custom sized robot. Two motor adaptor plates are provided with hardware to mount either IG32 or IG42 inline planetary gear motors to the chassis side.
These are not intended for our right angle gear motors, different adaptor plates would be needed.
The width of the robot is up to you, it must be at least (the length the gear motors + 2" tube + 1/8" adaptor plate) x 2. A simple piece of plywood, aluminum, or acrylic can be used to tie the two side together or the sides can be bolted onto your own robot chassis.
The tube structure uses the bearings in the above-linked wheel kits to support the axle. The other side of the axle is supported by the motor. This provides a much stronger attachment for the wheels and the load is put on heavy-duty bearings and tubes instead of the motor. Direct coupling a wheel to the motor will just ruin a motor quickly. Below are pictures of some assemblies using the sides and wheels.
Components required to connect 2 encoders to a Roboclaw Motor controller.