The transitional properties of the liquid crystal dimers formed by two mesogenic groups linked by a flexible chain exhibit a pronounced dependence on the number of atoms in the spacer. Here we present the results of a theoretical calculation of the nematic-isotropic transition temperature, the entropy of transition, and the second rank orientational order parameters at the transition for two homologous series of cyanobiphenyl dimers. In one the alkyl spacer is attached directly to the mesogenic groups while in the other the alkyl chain is linked via ether units to the two cyanobiphenyl groups. At the heart of the molecular field theory is its parametrization based on the surface tenser approach. In this theory the potential of mean torque for each conformer is related to its topology by the surface tenser, which ensures that molecules are aligned such that their surfaces overlap to a maximal extent. The transitional properties predicted by the theory are found to depend in a sensitive manner on the geometry of the cyanobiphenyl group and that of its link to the alkyl spacer. By using realistic geometries and allowing for the various conformational states within the rotational isomeric state model the theory is able to account for the behavior of homologous series of dimers as the spacer length is changed, and for the dependence of the transitional properties on the nature of the link between the alkyl chain and the mesogenic groups.