The motor rating determines the amount of load that can be supported by the gear assembly. On a 100 pound load motor, that would be 50 pounds by the motor and 50 pounds by the support. It is always best to support your spit rod on both sides when possible, taking the weight off the motor.
The amount of food, baskets and other items including the spit, should total about 50% of the stated motor load, but can be as little as 20% or as much as 100%. This allows for other factors such as extreme temperatures or out of balance conditions.
The common shaded pole rotisserie motor is not a high torque type of motor. They are built for higher speeds and are very inexpensive to produce. They have no torque to speak of and, without a gear box can be stalled with your fingers. They are typically used in fans for example. By converting that speed through a high reduction gear train, the torque is magnified many thousand times.
How much food can you actually put on?
It is all about balance. If the food was perfectly balanced on the rod so that there was no "Heavy Side", then you could put 100 pounds or even more if the rod would hold it. This is almost never the case. The food will always be out of balance to some degree. If it is out a lot, then you may only be able to have as little as 20 pounds on the spit. If it is balanced fairly well then you can expect to do about 50 pounds or so.
The most important thing to consider is the balance of the spit. If the spit is not properly balanced then the motor will have excessive intermittent loads applied to the gear train and that will reduce the life of the unit. These motors are an exceptional value for rotisserie cooking; however they are not industrial gear head motors. It is imperative that you balance your spit before using. A few extra minutes spent on this operation will make a huge difference in the life of your unit. A perfectly balanced heavy spit can be turned with your fingers easily. An out of balance spit, of the same weight may need a wrench. You should load your spit and test the spinning in your hands. If you can spin it easily and you have good balance, it is ready to grill.
The counterbalance weight is to make up for a minor out of balance condition. The easiest way to set it is to let the heavy side get to the bottom of the rotation. This is easy to see and hear, and is a natural condition for the load to want to be in. Turn off the motor. Then have your counterbalance weight pointing straight up. This will be exactly opposite the heaviest side.