  The Euclidean path integral method is applied to a quantum tunneling model
which accounts for finite size ($L$) effects. The general solution of the Euler
Lagrange equation for the double well potential is found in terms of Jacobi
elliptic functions. The antiperiodic instanton interpolates between the
potential minima at any finite $L$ inside the quantum regime and generalizes
the well known (anti)kink solution of the infinite size case. The derivation of
the functional determinant, stemming from the quantum fluctuation contribution,
is given in detail. The explicit formula for the finite size semiclassical path
integral is presented.
