It is important for a dogs protection to prevent jarring occurring whenever they work in harness (apart from weightpull, which has different techniques).
The size indicates the strength of the bungee as oppose to the length.
Small is for 1-3 dog teams; the medium is for 4-8 dog teams.
A bungee or shockline is an integral part of any working kit and takes different forms depending on the activity you are pursuing. If you are running a four dog husky team on a sled, you will require a different strength bungee to running a single collie in canicross; conversely a large malamute team would need an extremely heavy duty shock.