The effect of silicon nanowire (Si NW) diameter on the functionalization efficiency as given by covalent Si-C bond formation is studied for two distinct examples of 25 +/- 5 and 50 +/- 5 nm diameters (Si NW25 and Si NW50, respectively). A two-step chlorination/alkylation process is used to connect alkyl chains of various lengths (C-1-C-18) to thinner and thicker Si NWs. The shorter the alkyl chain lengths, the larger the surface coverage of the two studied Si NWs. Increasing the alkyl chain length (C-2-C-9) changes the coverage on the NWs: while for Si NW25 90 +/- 10% of all surface sites are covered with Si-C bonds, only 50 +/- 10% of all surface sites are covered with Si-C bonds for the Si NW50 wires. Increasing the chain length further to C-14-C-18 decreases the alkyl coverage to 36 +/- 6% in thin Si NW25 and to 20 +/- 5% in thick Si NW50. These findings can be interpreted as being a result of increased steric hindrance of Si-C bond formation for longer chain lengths and higher surface energy for the thinner Si NWs. As a direct consequence of these findings, Si NW surfaces have different stabilities against oxidation: they are more stable at higher Si-C bond coverage, and the surface stability was found to be dependent on the Si-C binding energy itself. The Si-C binding energy differs according to (C1-9)-Si NW > (C14-18)-Si NW, i.e., the shorter the alkyl chain, the greater the Si-C binding energy. However, the oxidation resistance of the (C2-18)-Si NW25 is lower than for equivalent Si NW50 surfaces as explained and experimentally substantiated based on electronic (XPS and KP) and structure (TEM and HAADF) measurements.