Experimental results for the dependence of highly oriented pyrolytic graphite (HOPG) conduction ESR (CESR) line shape and in 2(nd) stage graphite intercalation compounds (GICs) with nitric acid (C10HNO3) on the sample sizes are presented. Both in HOPG and in GICs the theoretical description of these dependences is possible only if a certain probability of the reorientation of current carrier spins during their collisions with the sample surface is taken into account. The significant increase of the graphite CESR linewidth at the intercalation of the narrow HOPG plate by HNO3 points out a nonzero probability of spin reorientation also during the collisions of electrons with the interface between the intercalated and the non-intercalated parts of the sample.