CO2 passes through rubber inner tubes much more readily than either N2 or O2, but the effect is due to chemical effects, not the size of the molecules. As stated by the OP, the CO2 molecules are largest of these three but still diffuse through more readily. CO2 consists of carbon with two double bonds to oxygen atoms. When this comes in contact with the carbon compounds of the rubber tube it can readily form a temporary bond to them - in effect sticking to the rubber molecule. That makes it more likely that it will eventually migrate through the rubber and escape to the outside air. OTOH, N2 molecules just bounce off the inner surface of the inner tube and are much less likely to find their way through.