We derived an analytical formula for the optimum width of the access waveguides of 1×N multimode interference (MMI) couplers. Eigenmode-decomposition-based simulations show that the optimum width relation corresponds to the points of diminishing returns in both insertion loss and output uniformity versus access waveguide width. We fabricate and characterize 1×12 MMI couplers on a nanomembrane of silicon-on-insulator substrate. The experimental investigations demonstrate that the analytical results can be reliably used as a design rule for MMI couplers with large number of outputs.