Fast reconnection is crucial to magnetospheric substorms, solar and stellar flares and fusion plasmas. Ultimate confirmation of fast reconnection must be achieved by multi-spacecraft detections of the reconnection rate itself and associated dimensions of the diffusion region. Here we report a multi-spacecraft measurement of fast reconnection rate γ rec ~ V in ≈ (0.07--0.15)V A based on directly measurements of the plasma flow into the diffusion region, where V in is the speed of reconnecting flux and V A the characteristic Alfv¿n speed. It falls in the range of ~(0.03--0.2)V A predicted in steady state reconnection simulation. The characteristic sizes for the diffusion region of the width L z ~ 0.9 d i (= 460 km) and the length L x ~ (3.3--5.1)d i (= 1680--2597 km) are measured as well. The length of the diffusion region L x is determined for the first time based on the in situ observations. Furthermore, other features detected during the event also match the previous observation and simulation results.