  We determine the density expansion of the radius of gyration, of the
hydrodynamic radius, and of the end-to-end distance for a monodisperse polymer
solution in good-solvent conditions. We consider the scaling limit (large
degree of polymerization), including the leading scaling corrections. Using the
expected large-concentration behavior, we extrapolate these low-density
expansions outside the dilute regime, obtaining a prediction for the radii for
any concentration in the semidilute region. For the radius of gyration,
comparison with field-theoretical predictions shows that the relative error
should be at most 5% in the limit of very large polymer concentrations.
