We experimentally study the radii of excitons in hBN-encapsulated WS(2 )monolayers by means of magneto-optical reflectance spectroscopy at cryogenic temperatures in magnetic fields up to 29 T. We observe field-induced energy shifts of the exciton ground and excited states due to valley Zeeman and diamagnetic effects. We find the g factor of the first excited state of -4.2 +/- 0.1 to be essentially equal to that of the ground state of -4.35 +/- 0.1. From diamagnetic shifts, we determine the root mean square radii of the excitons. The radius of the first excited state is found to be 5-8 nm and that of the ground state around 2 nm Our results further confirm the Wannier-Mott nature of the exciton quasiparticles in monolayer semiconductors and the assignment of the optical resonances in absorption-type measurements. They also provide additional support for the applicability of the effective mass hydrogenlike models in these systems.