This is measured by dynamic light scattering (DLS). DLS output the diffusion coefficient of some particles in the solvent, which might be proteins or polymers.
Then use Stokes-Einstein equation, you can calculate the Radius. That Radius is hydrodynamics radius.
Although the measurement of DLS assumes the particle is hard sphere, it is not in reality. The shape of the proteins or polymers and solvent around them will affect the diffusion coefficient. So the value got from DLS should be hydrodynamics radius.
Hydrodynamics radius considers the geometric effect and the water dynamics effect, so the value is larger than that of radius of gyration. The ratio between RG and RH should be around 0.7-0.8. The calculation of RH in MD simulation can be found at the following paper.
Lee, H., Venable, R. M., MacKerell, A. D. & Pastor, R. W. Molecular Dynamics Studies of Polyethylene Oxide and Polyethylene Glycol: Hydrodynamic Radius and Shape Anisotropy. Biophys J 95, 1590–1599 (2008).