Basically, the smaller the pinion the more torque the motor generates at the wheels, but at reduced speed. You can try playing around with a 21 speed bicycle to get a feel for it.

So if you decrease the size of the pinion, the motor doesn't have to draw as much current and it doesn't get as hot, but you won't go as fast. Increasing the pinion allows you to go faster, but the motor will get hotter since it has to push more current to achieve the same torque at the wheels.

The gearing depends on the type of vehicle you have and the motor. If you post that info, I think people on here can tell you what they have found to work.