Succah 3ab

And [a succah] that accommodates only one’s head and most of [his body], Beis Shammai rule invalid, while Beis Hillel rule valid.

Who taught this Baraisa that the Rabbis taught? A HOUSE THAT DOES NOT CONTAIN FOUR AMOS BY FOUR AMOS IS EXEMPT FROM THE MEZUZAH AND FROM FENCE; AND IT IS NOT CONTAMINATED BY tzaraas AFFLICTIONS; AND IT IS NOT IRREDEEMABLY SOLD AS ARE THE HOUSES OF WALLED CITIES; AND WE DO NOT RETURN ON ITS ACCOUNT FROM THE WARRIORS; AND WE DO MAKE AN ERUV WITH [SUCH A HOUSE], NOR DO WE MAKE A SHITUFEI WITH IT; AND WE DO NOT PLACE AN ERUV chatzeiros IN IT; AND WE MAY NOT MAKE [SUCH A HOUSE] A PROTRUSION BETWEEN TWO CITIES; AND BROTHERS AND PARTNERS DO NOT DIVIDE [SUCH A HOUSE].

For the next number of lines the Gemara explains the reasons behind this Baraisa.

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What follows are related rulings to the issue that a succah more than 20 amos high is invalid.

If [a succah] was more than twenty amos high and one came to reduce [its height] by mattresses and cushion, it is not a valid reduction.