Tahar and others in the Dresselhaus group previously reported a size effect in the electrical resistivity of benzene-derived carbon fibers with small diameters. A simple theory indicates that the resistivity has a linear dependence on the reciprocal diameter associated with boundary scattering together with a constant term due to a size-independent scattering process. The observed results are well represented by this relation if the diameter is large enough, while for samples with small diameter, the observed resistivity data are considerably smaller than the theoretical expectation. This discrepancy is removed by considering the correct boundary conditions and by solving the Boltzmann equation. Keywords Carbon fibers, Size effect of carbon fibers, Electrical resistivity of carbon fibers.