How big is the arena? How fast are you going? What difference does it make, just crash into my wall!
The bad pun in the heading just says that the Metric System is used internally. Speed is measured in Meters per Second (m/s), distance is measured in Meters (m), and so forth. There are those who say that these are bad units to use. Those folks are invited to write their own simulation without using meters, and then come back and make recommendations. :)
The standard grid is 500x500m. So the perimeter is 2000 meters, and the area is 250,000 square meters. By adjusting SIZE_FACTOR, you can change how big the grid is. This will change only the size of the arena! It will not affect the length of cycle walls, or their speeds, or anything like that. 30m/s is 30m/s no matter how big the grid is. So if you play on a smaller arena, the same speeds will feel faster than they are just because there is less room in which to maneuver.