1. We can use the principle of moments to ensure that there is no net moment, that is, m*g*d = M*g*l, where m is the mass of the weight, d is the distance of the weight to the pivot, M is the mass of the counterweight, l is the distance of the counterweight from pivot, and g is the gravitational constant.
2. A question that could be asked is what kind of weights is the engineer expecting the crane to lift, that would determine the counterweights used.