Previous studies of solution properties of dialkyl-substituted polysilanes have been measured in good solvents where long-range excluded volume interactions are present. Comparison of the experimental data on the polymer dimensions with theoretical predictions requires dimensions unperturbed by these long-range interactions. The mixed solvent of 2-propanol and hexane (41.3 wt % iPrOH) is shown to be a macroscopic THETA-solvent for poly(di-n-hexylsilane) at room temperature. Both the root-mean-square radius of gyration and the Stokes radius have been measured for a series of five molecular weights. The intrinsic viscosity in this mixed THETA-solvent has also been determined for seven molecular weights. These direct measurements of unperturbed dimensions are compared with previous estimates obtained by extrapolation to low M where long-range excluded volume interactions are minimized. The ratio of the thermodynamic and hydrodynamic dimensions is compared with those reported for more flexible polymers.