I would think 55mm? I'm just getting into reloading myself, but I would think the overall cartridge length should be what the rifle was designed for. I'm getting a 1913 M96 soon and haven't started reloading the 6.5x55 yet.
Please keep in mind that I'm a novice at reloading, however, I have reloaded 6.5x55 Swede ammo using 140gr. Spitzer BT bullets with an overall cartridge length of 3.20" and they seem to shoot well. The trim length was 2.155". Good Luck!