  It has been suggested that the volume dependence of the spectral weight could
be utilized to distinguish single and multi-particle states in Monte Carlo
simulations. In a recent study using a solvable model, the Lee model, we found
that this criteria is applicable only for stable particles and narrow
resonances, not for the broad resonances. In this paper, the same question is
addressed within the finite size formalism outlined by L\"uscher. Using a
quantum mechanical scattering model, the conclusion that was found in previous
Lee model study is recovered. Then, following similar arguments as in
L\"uscher's, it is argued that the result is valid for a general massive
quantum field theory under the same conditions as the L\"uscher's formulae.
Using the spectral weight function, a possibility of extracting resonance
parameters is also pointed out.
