Nominally doped and undoped nano- and microsized Silicon powders were characterized by determining in-situ the conductance, impedance, and the change of porosity while applying a uniaxial mechanical pressure ranging from 7.5 to 750 MPa. The conductance shows an exponential dependence on the applied pressure for nanosized particles and a power law for microsized particles. Simple scaling considerations with respect to the particle size cannot explain this fundamentally different behavior. A slow time dependent change in conductance together with a decrease in porosity was observed while applying a constant pressure, suggesting friction-limited compaction of the powder. For a constant external force, the comparison of samples with different particle size leads to a clear power law dependence between the conductance of pressed samples and their mean particle diameter. This size effect spans seven orders of magnitude in conductance while the particle size changes by only a factor of ten. The conductance clearly exceeds any influence of the doping concentration and the variation of the sample mass. In agreement with the observed compaction of the powder, impedance spectra show a strong increase of the sample capacitance and conductance as a function of the applied pressure.