Summary: The finite size theory of metastability in a quartic potential is developed by the semiclassical path integral method. In the quantum regime, the relation between temperature and classical particle energy is found in terms of the first complete elliptic integral. At the sphaleron energy, the criterion which defines the extension of the quantum regime is recovered. Within the latter, the temperature effects on the fluctuation spectrum are evaluated by the functional determinants method and computed. The eigenvalue which causes metastability is determined as a function of size/temperature by solving a Lamé equation. The ground state lifetime shows remarkable deviations with respect to the result of the infinite size theory.