The number of rotors was limited by the keyboard effort. All those spring-loaded contacts between the rotors make advancing the rotors hard. Enigmas have a huge key travel, almost an inch, to provide extra leverage. You can't actually "type" on one; you push the keys down firmly, one at a time. Somebody has to write down which lamp came on, anyway, so that's not the limit on speed.<p>The keyboard-driven rotor machines didn't scale up well. Beyond three rotors, motor drive was usually needed, although the cute little M-209 used a wheel and lever for input, rather than a keyboard. The next step up was the Lorenz SZ-40/42, a six-rotor machine set up as a peripheral for a Teletype. That was cracked with the Colossus electronic key-tester.<p>The Germans built the T-52 Geheimschreiber, with 10 rotors, but that had a built-in Teletype machine and was too heavy to be portable. The Allies didn't crack that one.<p>That was the original limit on key length. The longer the key, the bigger and heavier the machine.