The Io footprint (IFP) is the auroral signature of the electromagnetic interaction between Io and Jupiter's magnetosphere. It consists of several spots followed by an extended tail, which are located close to the feet of the magnetic field lines connecting Io to Jupiter. The size of the main spot is a controversial issue, and previously published values range from ~400 to ~8000 km. However, this question is crucial to understand the processes at play, since this quantity is expected to reflect the size of the interaction region at Io. The present study provides estimates of the size of the Io footprint on a much larger image sample than before, paying a particular attention to the differentiation of the spots and to their 3-D structure. The length of the Main Alfven Wing (MAW) spot and the length of the trans-hemispheric electron beam (TEB) spot along the footpath are similar to 850 km, while their width perpendicular to the footpath is < 200 km. Larger lengths are sometimes observed, but these configurations may be attributed to the overlaps of the different spots. The spot lengths are larger than the projected diameter of Io along unperturbed magnetic field lines, which is consistent with recent simulations. The narrowness of the IFP will need to be carefully accounted for in future studies of its brightness. Additionally, the peak altitudes of the MAW and the TEB spot are as high as 900 and 700 km, respectively, which seem to confirm their different origins
