Small robots , 1 to 3 pounds depending on.wheels used and drag. If your concerned nd want to.use servos , stop by pololu.com and get power HD 1501mg servos and modify them for continuous rotation (its easy). Those servos are 20 bucks but have 240 oz in torque (ez robot servos are about 45 oz), plenty to move around a small to med size bot. Maybe 15 pounds or so , that kind of torque is a lot.

if using 2 servo's it doubles the carrying torque any servo can turn it to a continuous servo,just by removing the pot and adding two 2.5 k resistors 5k divide by 2 = 2.5 k ,you need the pot in the center,can leave the pot in,but need to adjust it to center,second need to remove the plastic or metal pin on the gear attach to pot much online info on how to do this blush

Your right about torque being doubled as long as both servos are running , some people use two wheel skid steering though , nice info on the resistors , I need some of those.

two wheel skid steering is still the same,torque will be doubleonly if using one servo with 2 wheels torque will not be double much much info on the internet on how to modify a servo this one here has a schematic of a common S3003 servo MODIFY A S3003 SERVO

but main link to modify it for continuous operation

MODIFY A servo for continuous

also about resistors doesnt need to be 2.5k ,just 2 matched near that value so the voltage is at half point for the comparator in the chip can use two 5k too