  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
Lam\`{e} equation. The ground state lifetime shows remarkable deviations with
respect to the result of the infinite size theory.
