We formulate an approach to the size effect problem in ferroelectric-electrode systems where ab initio calculations are combined with a phenomenological framework. The parameters of the model can be extracted from the calculations, while experimentally verifiable predictions can be made on thick films. We illustrate this approach by applying it to SrRuO3/BaTiO3/SrRuO3 heterostructures with different terminations of the metal-oxide electrodes.