Barry,

I believe you are asking a contradiction between Rashi 2a DH Sukah mid'Oraisa, who writes that the maximum height of the Sukah is determined by a Halachah l'Moshe mi'Sinai, and the Gemara on bottom of 4b, which asks how we know the minimum height of a Sukah is 10 Tefachim. Why not answer that it is a Halachah l'Moshe mi'Sinai (HLM) as well?

By now, you must have reached Daf 5b, and seen that the Gemara indeed does conclude that at least according to Rebbi Yehudah the minimum height is indeed learned from a HLM. If so, the Gemara is just saying that according to Rebbi Meir, the HLM is not necessary for teaching the minimum height; it can be learned from the Keruvim or Aron. The Gemara only asked "Menalan" before it knew that the laws of Mechitzos are learned from HLM.

However, your question still applies to the conclusion of the Gemara on Daf 6b. There, the Gemara asks what the HLM of Mechitzos teaches according to Rebbi Meir, and answers "Gud, Lavud, and Dofen Akumah." What about the maximum height of the walls? The answer to that is that Rashi on Daf 2a, who mentions "mi'Sinai," does not seem to be referring to a HLM at all. In fact, he cannot be referrring to a HLM, since the Gemara a few lines later gives three different sources from verses that show the maximum height is 20 Amos (each Amora learns this from a different verse). Rather, Rashi means that the height of a Sukah was known from the time of the giving of the Torah, through the verses of the Torah, and not from a HLM.

Best wishes,

-Mordecai