Single molecules set the ultimate miniaturization limit of conductive components in electronic circuits. A major challenge of molecular electronics is to achieve high and robust conductivity. However, it is not clear what is the upper boundary for conductance across a single molecule and what are the factors that would determine this limit. We addressed these questions by studying the effect of molecule length on the conductance in metal-molecule-metal junctions, based on a series of oligoacene molecules. We found that the conductance can reach an upper limit where it is independent on molecule length. Furthermore, we found that this limit can be controlled by rational orbital hybridization at the metal-molecule interface. Interestingly, the evolution of conductance towards its upper limit can be understood with the aid of a simple analytical model. Our findings shed light on the mechanisms that determine the upper limits for conductance across molecules, providing guiding principles for the design of highly conductive metal-molecule interfaces.