The thermal stability of coupled trilayer synthetic antiferromagnets is investigated in the framework of the new model, taking into account of the magnetostatic fields. At magnetic and other parameters typical for magnetic random access memory applications, the thermal stability of magnetic cells becomes problematic as the lateral dimensions approach 150 nm. At these dimensions, the energy barrier at zero applied field is calculated to be 40 kT (at room temperature) and it decreases with increasing applied field. The effects related with the shape anisotropy, which are expected to be strong in this size range, are not properly described by the existing model. The present results show that this problem can be solved by accurately quantifying the magnetostatic fields.