If a drive axle or other axle is currently selected, you can specify its total length in inches. The length you specify should be enough to go all the way through the car body, extend through the wheels on both sides, and allow enough extra length to (1) provide clearance between the car body and the wheels, and (2) to provide room for you to grab and rotate the drive axle(s) when you are winding up the car.
If the spring is currently selected, you can specify its length. The length of the spring plays a large role in determining how far the car will travel, because a longer spring delivers more revolutions of the drive axle as it unwinds from its wound state. But, the longer the spring, the larger the spring chamber must be to accommodate it! This is because the spring needs space to unwind into, and the longer the spring, the more space is needed. If you choose a very long spring, you will find that the spring chamber will become much larger than you want.