  We study the finite-size behavior of the low-lying excitations of spin-1/2
Heisenberg chains with dimerization and next-to-nearest neighbors interaction,
J_2. The numerical analysis, performed using density-matrix renormalization
group, confirms previous exact diagonalization results, and shows that, for
different values of the dimerization parameter \delta, the elementary triplet
and singlet excitations present a clear scaling behavior in a wide range of
\ell=L/\xi (where L is the length of the chain and \xi is the correlation
length). At J_2=J_2c, where no logarithmic corrections are present, we compare
the numerical results with finite-size predictions for the sine-Gordon model
obtained using Luscher's theory. For small \delta we find a very good agreement
for \ell > 4 or 7 depending on the excitation considered.
