The electron dynamics in metallic nanoparticles subjected to fs pulses shows distinctive features as a function of size. By reducing the size from approximate to 20 down to approximate to 1 nm, we progressively go: (i) first below the light penetration depth (approximate to 13-14 nm) and then (ii) below the mean free path (approximate to 5 nm). This affects drastically the behaviour of the electron dynamics in nanoparticles. More precisely, we observe: with (i) the disappearance of coherent acoustic oscillations generated by the fs pulses and clearly observable at large sizes; with (ii) the size dependence of the electron thermalization inside the nanoparticles. The above mentioned effects have been studied both in the solid and liquid state of the nanoparticles, In order to observe and study such phenomena it is essential that the nanocrystals are grown in a wide size range, with a regular (nearly spherical) shape and a relatively low size dispersion.