  We study the finite size corrections for the magnetization and the internal
energy of the 2d Ising model in a magnetic field by using transfer matrix
techniques. We compare these corrections with the functional form recently
proposed by Delfino and LeClair-Mussardo for the finite temperature behaviour
of one-point functions in integrable 2d quantum field theories. We find a
perfect agreement between theoretical expectations and numerical results.
Assuming the proposed functional form as an input in our analysis we obtain a
relevant improvement in the precision of the continuum limit estimates of both
quantities.
