Abaye mentions 8 Tannaim that hold that a sukkah must be Keva - which includes R' Yehudah. The Gemarra brings a proof to R' Yehudah from the first Mishnah. R' Yehudah holds a sukkah above 20 amos is kasher. - How does this prove that R' Yehudah holds that a sukkah must be Keva? He allows a sukkah to be over 20 amos, but where do we see that he holds it must be ÷??
Avraham Sacks, Ramat Beit Shemesh
There are two opinions. The Tosfos ha'Rosh (7b, DH Kulhu) learns that Rebbi Yehudah allows even Keva (contrary to the simple understanding of 7b).
However, the Ritva says that even though this Mishnah is no proof to allow only Keva, there are proofs from other Mishnayos to Rebbi Yehudah, such as on 20b, where Rebbi Yehudah permits sleeping under a bed in a Sukah, since the bed is Arai and the Sukah is Keva. (The Rosh refutes this proof.) This is also the view of the Ramban (Milchamos, DH Kasuv) and the Me'iri (DH ul'Inyan Psak).
See Mesivta edition, Sukah 7b, comments 12 and 15, and 2a, comment 10.
All the best,
Reuven Weiner