  We consider diagonal matrix elements of local operators between multi-soliton
states in finite volume in the sine-Gordon model, and formulate a conjecture
regarding their finite size dependence which is valid up to corrections
exponential in the volume. This conjecture extends the results of Pozsgay and
Tak\'acs which were only valid for diagonal scattering. In order to test the
conjecture we implement a numerical renormalization group improved truncated
conformal space approach. The numerical comparisons confirm the conjecture,
which is expected to be valid for general integrable field theories. The
conjectured formula can be used to evaluate finite temperature one-point and
two-point functions using recently developed methods.
