Siman 650

Measuring the length of the lulav is not similar to measuring the length of the hadas and aravah, for with them one measures from the end of the wood until the top. However, with the lulav one measures from its end, and not until the top but rather until the end of the spine, which is to say, until the place where the leaves are attached to the spine. The leaves from there and further, which do not have a spine—they are not included in the calculation that will be explained. And thus the halacha was ruled in the Gemara (32b). And one should not wonder regarding the fact that one measures from their actual end: is there not no leaves there at all? For this is no wonder, since in the majority of their length there are leaves—it requires no more. And the Torah said "a thick tree" regarding the hadas—behold the essential part is the wood, and consequently one measures the entire wood in all of them. And because of this reason it appears that one does not measure in the lulav from the spine and further up, because from there and further it is not wood, but rather leaves. Therefore also with the hadas and aravah, if the leaves extend above the wood—nevertheless the measurement is only until the end of the wood, and not with the leaves that rise above the wood.

The Rambam wrote in the seventh chapter, law 8: What is the measure of the length of each species among them? A lulav is no less than four tefachim, and its measurement is from its spine alone, not from the tips of the leaves. And the hadas and aravah—no less than three tefachim. Thus far his words, and the Rif wrote similarly. According to this, the length of the lulav is sixteen thumb-breadths, and the length of the hadas and aravah is twelve thumb-breadths. Our teachers have already wondered at their words, for in the Gemara there it is explicitly explained that the measurement is with an amah of five tefachim, and it follows according to this that the lulav must be thirteen and a third thumb-breadths, and the hadas and aravah ten thumb-breadths. This is the opinion of the Rosh, the Tur, the Ra'avad, and all our teachers. And such is the primary ruling. Only the opinion of the Ramban, of blessed memory, is: that even though the hadas and aravah are according to the calculation of an amah of five tefachim—and if so, the Mishnah which required three tefachim for their measure, they are ten thumb-breadths—nevertheless, the tefach that the lulav exceeds them is like a standard tefach of four thumb-breadths, and according to this, the lulav is fourteen thumb-breadths until the spine.

And for the law: certainly if one is able to fulfill the mitzvah in the optimal manner according to the words of the Rif and the Rambam—it is well. However, the primary ruling is according to the majority of the poskim, as I have written. And know: that if the hadas and the aravah are very large, nevertheless the spine of the lulav must be a tefach higher than them. And there is no limit to their size; even if they are very long—they are kosher.