[Animats]

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.

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.

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.

That was the original limit on key length. The longer the key, the bigger and heavier the machine.