"Error: size constraint hasn't been set up"

This error can occur if you are solving for both a moving object and moving camera. You need to have a size constraint for the camera (one way or another), and a size constraint for the object (one way or another). So you must have TWO size constraints, one for each object.

It isn't immediately obvious to many people why TWO size constraints are needed. This is the basis of a well-known optical illusion, relied on in shooting movies such as "Honey, I Shrunk the Kids". Basically you can't tell the difference between little things up close, and big things farther away. You need both size constraints to set the relative proportional size of the foreground (object) and background (camera).