Proton radii of ^{12-19}C densities derived from first accurate charge changing cross section measurements at 900A MeV with a carbon target are reported. A thick neutron surface evolves from ∼0.5 fm in ^{15}C to ∼1 fm in ^{19}C. The halo radius in ^{19}C is found to be 6.4±0.7 fm as large as ^{11}Li. Ab initio calculations based on chiral nucleon-nucleon and three-nucleon forces reproduce the radii well.