The structure sensitivity and cluster size convergence for formate adsorption on the Cu(100), Cu(110) and Cu(111) surfaces have been investigated systematically using density functional theory and the cluster model containing up to 40 Cu atoms. The copper core-valence correlation effect on the adsorbate-surface interaction is examined by using three different basis sets and effective core potentials. The calculated geometries and vibrational frequencies are in good agreement with experimental data even on the small clusters and are not surface sensitive. However, the adsorption energies show strong dependence on the surface structure and the cluster size. The adsorption energies are shown to converge very well for the large clusters, and the activity of the Cu planes for formate adsorption is in the order of Cu(110)> Cu(100)> Cu(111), the same as that observed experimentally for methanol synthesis. Regardless of the basis set, cluster size and surface structure, all results show an anionic formate adsorption species. The chemisorption mechanism and the local structure of formate on the three copper surfaces are essentially very similar. Some discussion about cluster modeling is presented.