If 20 Amos is Halacha l’moshe misinai how can R. Yehudah dispute the Rabbis?
The gemoro at the beginning of Succa discusses many possibilities for the 20 amos height maximum for succa.
While many other shiurim are halacha lemoshe misinai, height of succa is not one of them.
There are numerous issues that there are a machlokes whether or not a law is halacha lemoshe misinai or not.
Please elaborate on your question. What axiom or assumptions you are making that lead you to have a question.