The characterization of a bearing fault and its size is a first important stage towards a reliable model based prognostics, leading to an assessment of the remaining useful life of the faulty bearing. This paper describes the first stage in a research aimed to propose an approach for model-based prognostics. The approach utilizes various methods of vibration analysis and vibration based condition indicators which proved to be effective for detecting bearing degradation. While the problem of detecting a fault is more researched, the relationship between the health indicators (aggregated condition indicators) and the size of the damage, as well as the modelling of evolving damages are less researched and less understood. As a result, the ability to assess the severity of the damage or predict the remaining useful lifetime of a damaged bearing is limited. The current research is aimed to advance the knowledge in this difficult problem. A 3D ball-bearing model that enables a simulation of its dynamics under the influence of a wide spectrum of faults was used. The goal was to study and predict the influence of the bearing fault size and the load on the generated vibration pattern. Based on the obtained insights, several health indicators were considered and compared. The relationship between the fault size and the selected health indicators will be demonstrated by presenting results from experiments on a test fixture with seeded faults of different sizes and different loads, as well as the corresponding outcome of the dynamic model of the bearing.