In order to study the size of the chains in binary polymer blends we determine the mean-square end-to-end distance of the polymer coils of the A or B kind. Chains A and B interact generating the one-loop diagrams, which are summed up to all orders of concentration and interaction. Interactions that do not favor mixing are found to reduce the size of the macromolecules producing a deviation from the unperturbed dimensions of the order of 1/root N. The effect is stronger on chains of the smaller concentration whereas chains of large concentration and of large molecular weight, N-->infinity, tend to reach an ideal Gaussian behavior. Chains A are found to be more contracted when chains B are small acting as a bad solvent. As the system comes close to the phase separation limit, chains shrink obtaining a smaller size on the spinodal, where a second-order phase transition is found. Our results are in accord with relevant SANS small angle neutron scattering and Monte Carlo experiments.