  We experimentally study the radii of excitons in hBN-encapsulated WS2
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.
