פירוש על שבת 43:1

Invalid: Since the eye will not master (notice) [things] above twenty cubits, and there will be no publication of the miracle.

A Hanukkah lamp that one placed above twenty cubits is invalid, just as a sukkah and as an alleyway: But perhaps you will say, "It should have said, 'Reduce [the height of] it,' like regarding the alleyway. Since there (Eruvin 2a), it said, 'an alleyway is rabbinic - [that is why] they taught the rectification.'" But it can be said (answered) that it mentioned, "invalid," because it needed to say, "just as a sukkah," which is from the Torah. Or also (another explanation) is that it did not want to speak at length: "Extinguish it, reduce [the height of] it and light it." As to reduce it and leave it as it is, would not be possible - as is shown later (Shabbat 23a).

Sukkah and alleyway: Since we learned (in the Mishnah) explicitly in Eruvin and in Tractate Sukkah, that they are invalid.