The determination of the extinction in different directions in the Galaxy and in external galaxies is of paramount importance in several fields of astrophysics. The determination of the extinction law provides clues to the nature of the Inter/Circum-Stellar Medium (ISM/CSM), specifically to that of the dust particles. Recent studies have shown that the extinction curves toward a few, highly-reddened SN Ia have values of RV , the ratio of total-to-selective extinction, significantly lower than the canonical 3.1. Whether this is the case only for high extinction objects is not clear yet. Clarifying this is extremely important both for understanding the nature of the supernovae (SNe) and for the calibration of SNe Ia as cosmological standard candles. Thus, in the context of our present knowledge of dust, we discuss here the information that we can obtain from these cases of highly extinguished SNe Ia, i.e. the possibility of deriving the average size of the dust grains along their lines-of-sight, which, in all cases, is smaller than the average dust grain size found in the Galaxy, and offer a complementary study of the behaviour of the NaID absorption lines in the SNe spectra. If the interstellar NaID doublet is present in the spectra we find that the supernova light is at least reddened by E(B - V ) = 0.11 × EW(Na ID).