The energy spectrum of two stable particles in a finite volume is shown to be expandable in an asymptotic power series of 1/L. The coefficients of these expansions are related to the elastic scattering amplitude in a universal manner. At low energies, the scattering amplitude can be determined if accurate two-particle energy values are known. This paper continues previous work on the size dependence of stable particle masses in quantum field theories. The objective is to understand how energy eigenstates of two stable particles behave in finite volume and how their energy values depend on the volume. This is important for numerical simulations, as it helps in spectral analysis and interpreting energy spectra. The paper also shows that the size dependence of two-particle energies is related to elastic scattering amplitudes, making them accessible for calculations that require finite volumes. In finite volume, particle momenta are quantized, leading to discrete energy levels for two-particle states with zero total momentum. As the volume increases, the spacing between these levels decreases, but in practice, the spacing can be significant. For example, in QCD simulations, the low-lying energies of ππ states are approximately given by the free field expression. These energy values are well-defined and can be calculated in numerical simulations. In a general massive quantum field theory, the energy values of two-particle states are given by a free field expression plus a small correction due to interactions. Two processes contribute to this correction: polarization effects and direct meson interactions. For large volumes, polarization effects decrease exponentially, while direct interactions give corrections that decay as a power of 1/L. This is due to the short-range nature of interactions in massive quantum field theories.The energy spectrum of two stable particles in a finite volume is shown to be expandable in an asymptotic power series of 1/L. The coefficients of these expansions are related to the elastic scattering amplitude in a universal manner. At low energies, the scattering amplitude can be determined if accurate two-particle energy values are known. This paper continues previous work on the size dependence of stable particle masses in quantum field theories. The objective is to understand how energy eigenstates of two stable particles behave in finite volume and how their energy values depend on the volume. This is important for numerical simulations, as it helps in spectral analysis and interpreting energy spectra. The paper also shows that the size dependence of two-particle energies is related to elastic scattering amplitudes, making them accessible for calculations that require finite volumes. In finite volume, particle momenta are quantized, leading to discrete energy levels for two-particle states with zero total momentum. As the volume increases, the spacing between these levels decreases, but in practice, the spacing can be significant. For example, in QCD simulations, the low-lying energies of ππ states are approximately given by the free field expression. These energy values are well-defined and can be calculated in numerical simulations. In a general massive quantum field theory, the energy values of two-particle states are given by a free field expression plus a small correction due to interactions. Two processes contribute to this correction: polarization effects and direct meson interactions. For large volumes, polarization effects decrease exponentially, while direct interactions give corrections that decay as a power of 1/L. This is due to the short-range nature of interactions in massive quantum field theories.