How large are the halos of light nuclei?

We present quantitative reaction cross section calculations for halo nuclei at high energy which retain the essential few-body correlations in such structures. Inclusion of these correlations leads to larger deduced halo radii. For three-body (n+n+core) projectiles, cross section measurements determine the rms hyperradius of the system and, given the core nucleus size, the projectile matter radius. The importance of finite range effects are estimated in the case of the lightest two-neutron-halo system 6He. Outstanding uncertainties are discussed. (orig.)