Photo from the Sukkot Exhibit, curtesy of Neot Kedumim Park
(א) סֻכָּה שֶׁהִיא גְבוֹהָה לְמַעְלָה מֵעֶשְׂרִים אַמָּה, פְּסוּלָה. רַבִּי יְהוּדָה מַכְשִׁיר. וְשֶׁאֵינָהּ גְּבוֹהָה עֲשָׂרָה טְפָחִים, וְשֶׁאֵין לָהּ שְׁלֹשָׁה דְּפָנוֹת, וְשֶׁחַמָּתָהּ מְרֻבָּה מִצִּלָּתָהּ, פְּסוּלָה.
(1) A sukka, i.e., its roofing, which is the main and most crucial element of the mitzva, that is more than twenty cubits high is unfit. Rabbi Yehuda deems it fit. Similarly, a sukka that is not even ten handbreadths high, and one that does not have three walls, and one whose sunlight that passes through its roofing is greater than its shade are unfit.