דברי תורה עניים במקום אחד ועשירים במקום אחר. The Rambam himself, elsewhere in his writings (specifically in his Introduction to the Mishnah), carefully distinguishes between the details given to Moshe at Sinai, which are universally accepted, and the details that were derived using human analysis, some of which are subject to debate. He thus writes:
המשנה, שהיא כוללת פירוש כל המצות הכתובות בתורה. מהם קבלות מקובלות מפי משה ע"ה. ומהם דעות הוציאום בדרך הסברה ואין עליהם מחלוקת. ומהם דעות שנפלה בהם מחלוקת בין שתי הסברות...
וזה עיקר יש לך לעמוד על סודו. והוא, שהפירושים המקובלים מפי משה, אין מחלוקת בהם בשום פנים, שהרי מאז ועד עתה לא מצאנו מחלוקת נפלה בזמן מן הזמנים, מימות משה ועד רב אשי בין החכמים, כדי שיאמר אחד, המוציא עין חברו יוציאו את עינו, שנאמר עין בעין, ויאמר השני, אינו אלא כופר בלבד שחייב לתת...
אבל מי שיחשוב שהדינים שנחלקים בהם כמו כן מקובלים מפי משה, וחושבים שנפלה המחלוקת כדרך טעות ההלכות, או השכחה, או מפני שאחד מהם קיבל קבלת אמת והשני טעה בקבלתו... זה הדבר מגונה מאוד, והוא דברי מי שאין לו שכל ואין בידו עיקרים, ופוגם באנשים אשר נתקבלו מהם המצות, וכל זה שווא ובטל.
...the Mishnah, which contains the explanations of all of the mitzvos written in the Torah. Some of these are explanations received from Moshe o.b.m., some were derived by logical analysis and are not debated, and still others are subject to debate...
You must be aware of the following subtle point: the explanations received from Moshe are not subject to debate. Never, from his time until now, was there ever any debate among the sages - from Moshe through Rav Ashi - where one claims that if A puts out B's eye, then they should put out A's eye (since it says "an eye for an eye") while another says it means just a monetary payment... (and continues with several other such examples).
Now, anyone who thinks that the debated laws were also received from Moshe, and believe that they became debatable because of mistakes in deciding the halachah, or forgetfulness, or because one learned the correct version and the other learned a mistaken one... - this is an abhorrent view, and would be held only by someone who has no intelligence and no basic principles. Such a person insults the people from whom we received the [explanations of] the mitzvos. The entire idea is baseless and worthless.
In fact, earlier in the same introduction the Rambam speaks of sukkah (the first example in the Eighth Principle), noting how its various details were given to Moshe and then transmitted throughout the generations to us.
Applying this to your question, then, "the form of Sukka, the Lulav, the Shofar, the Tzitzit, the Tefillin... are the actual forms which G-d told to Moses" - i.e., the basic parameters of each of these mitzvos that were never subject to dispute among the sages. They all universally agree, for example, that there is a mitzvah to take the lulav with the other species (not as the Karaites, for example, claimed, that the relevant verse is simply telling us to use these species to build the sukkah), and what the identities of those species are, and so forth. Likewise, they all agree that a shofar is a horn of an animal, and that it has to be blown in a certain series of sounds on Rosh Hashanah. That there are disputes about the finer details - what's the minimum size of an esrog? What exact sounds constitute a teruah? - doesn't contradict that principle, because the very fact that those details are debated shows that they were originally arrived at by human analysis rather than by transmission from Hashem to Moshe to us.
