Siman 650
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א שִׁעוּר הֲדַס וַעֲרָבָה, ג' טְפָחִים; וְשִׁדְרוֹ שֶׁל לוּלָב ד' טְפָחִים, כְּדֵי שֶׁיְּהֵא שִׁדְרוֹ שֶׁל לוּלָב יוֹצֵא מִן הַהֲדַס טֶפַח, בְּאַמָּה בַּת ה' טְפָחִים עָשָׂה אוֹתָהּ ו', צֵא מֵהֶם ג' לַהֲדַס, נִמְצָא שִׁעוּר הֲדַס וַעֲרָבָה טְפָחַיִם וּמֶחֱצָה, שֶׁהֵם י' גּוּדָלִים; וְשִׁעוּר שִׁדְרוֹ שֶׁל לוּלָב י"ג גּוּדָלִים וּשְׁלִישׁ גּוּדָל; וְיֵשׁ מִי שֶׁאוֹמֵר שֶׁשִּׁעוּר שִׁדְרוֹ שֶׁל לוּלָב י"ד גּוּדָלִים; וְיֵשׁ אוֹמְרִים שֶׁשִּׁעוּר הֲדַס וַעֲרָבָה י"ב גּוּדָלִים, וְשִׁדְרוֹ שֶׁל לוּלָב י"ו גּוּדָלִים וְכֵן נוֹהֲגִין לְכַתְּחִילָה.
1 The Required Measurements for the Myrtle and Willow, 2 Seifim: The minimum size for myrtle and willow is three tefachim. For the spine of the lulav, it is four tefachim, such that the spine of the lulav extends one tefach beyond the myrtle. [The tefach we are talking about can be gotten by] taking an amah of five tefachim and dividing it into six, using three for the myrtle. The measure of the myrtle and willow ends up being 2.5 [normal] tefachim, which is ten godels. The measure of the spine of the lulav is thirteen and a third godels. There is one who says that the minimum size of the lulav spine is fourteen godels. And there are those who say that the minimum size of the myrtle and willow is twelve godels and the minimum size of the lulav spine is sixteen godels (and we do this, initially).
ב אֵין לָהֶם שִׁעוּר לְמַעְלָה. וְיֵשׁ מִי שֶׁאוֹמֵר שֶׁאֲפִלּוּ הוֹסִיף בְּאֹרֶךְ הַהֲדַס וְהָעֲרָבָה כַּמָּה צָרִיךְ שֶׁיֵּצֵא שִׁדְרוֹ שֶׁל לוּלָב לְמַעְלָה מֵהֶם, טֶפַח.
2 They have no maximum size. There is one who says that if you add to the length of the myrtle and willow, the spine of the lulav must extend one tefach beyond them.