The counterbalanced TRX is pretty forgiving, most are reasonable with vibration to around the popular 363cc 76 stroke x 78 piston. But everyone has their own tolerance level and guys balance them as small as the 295cc.
But for the most part you are really not balancing them but infact changeing the balance factor, in the tech section there is a few writeups on crank balancing http://trx250r.org/forums/62-Engine-Bottom-End
Allow me to try to summarize or give a crash course in crankshaft balancing for those that do not want to read all of the stuff in the above link
1. We can static balance just about anything but it is an impossible engineering feat to achieve complete dynamic balance of any crank assembly that has less than four cylinders. Dynamic balance is possible with the proper crankshaft journal configuration that allows two piston and two connecting rods to have opposite and equal motions. Engines that have multiples of 4 cylinders (4, 8, 12 etc.) makes complete static and dynamic balance possible.
2. Three cylinder crankshafts can run smoother than twin cylinder crankshafts and twin cylinder crankshafts can runs smoother than single cylinder crankshaft when the comparing the vibrations due to imbalance.
3. We apply a balancer factors to single and twin cylinder crankshafts. If we statically balance a single or twin cylinder, the engine will shake in the horizontal plane 90 deg, to the piston motion.
4. Changing the balance factor of a single cylinder crankshaft changes the forces minimally that are trying to rip the crankshaft out of the cases but changing the balance factor has the largest influence on the direction that the engine shakes.
5. Adding a counter balancer shaft to a single or twin cylinder with properly positioned counter-weights reduces the overall engine shake but does not keep the crankshaft from trying to rip itself from the cases.
5. Engineers experiment with different balance factors to determine which balance factor excites a particular chassis design the least and provides the rider with the least amount of discomfort. I see some shops selling dynamic balancing services for single and twin crankshafts. These shops have not been properly trained or are taking advantage of their customers lack of understanding of the physics of not being able to dynamically balance their type of engine.
6. Twin cylinder crankshaft engines produce what is called a rocking couple. Simply stated, the center line of a running twin cylinder crankshaft traces the shape of an hour glass. The two end of the crank move in a circle and the point halfway between the cylinders is motionless. As the distance between cylinders and the mass of the pistons increase, the rocking couple forces increase exponentially.
