Your question is based on the Gemara (11a) which says that a Tzuras ha'Pesach works to make a valid Pesach even when the Pesach is higher than 20 Amos. If so, why doesn't the Mishnah at the beginning of the Masechta mention this, just like it mentions that a Tzuras ha'Pesach helps even if the opening is wider than 10 Amos?

We find on 2b that Rebbi Yehudah in the Beraisa argues regarding the width of a Pesach. In the Mishnah, though, he only argues regarding the height of a Pesach, yet the Gemara says "so, too, he argues on the width, in a Beraisa". If so, here, too, regarding Tzuras ha'Pesach, the Mishnah mentioned that it helps for an opening that is wider than 10 Amos, and "so, too," it helps for a height greater than 20 Amos, as explained in the Beraisa (11a).

It could be that the Gemara on 2b which says that Rebbi Yehudah, who argues in the Mishnah concerning the height, also argues concerning the width, means to say a "Kol she'Ken.: That is, if Rebbi Yehudah holds that the a height of greater than 20 Amos does not diminish the Heker, even though the Heker should be greatly diminished when it is above that height, then certainly he will hold that the width may be wider than 10 Amos (which does not diminish the Heker of the Korah). Similarly, if the Mishnah says that a Tzuras ha'Pesach works for a width greater than 10 Amos, even though that is considered an absolute Pirtzah, then certainly it will work for a height greater than 20 Amos (which is not considered a Pirtzah).

Kol Tuv,

M. KORNFELD