An innovative method is presented to estimate the size effect on the ferroelectric phase transitions in the barium titanate ceramic system. The method is based on the phenomenological thermodynamic theory with additional stress led by surface bond contraction. Two symmetry-breaking factors are introduced to correct the stress. This method has enabled us to predict the changing trends in phase transitions and the critical grain size for the disappearance of ferroelectric phase, which is beyond the prediction of existing models.