Length:[1] 

The length of the spine of the Lulav must be four Tefach. If the Hadassim or Aravos are more than three Tefach than the spine of the Lulav must be at least one Tefach higher than the Hadassim and Aravos.

How much is four Tefachim? Some Poskim rule that every Tefach is 4 Agudlin and hence four Tefachim is 16 Agudlin [32cm.] Others rules that each Tefach is 3.33 Tefach and hence four Tefach is 13.3 Agudlin [26.6 cm]. Practically we are stringent to follow the first opinion of 32 cm. However in a time of need one may be lenient to use a Lulav that has a spine of 26.6 cm. If one already used such a Lulav and then found one with 32 cm. he should shake it without a blessing being that he has already fulfilled his obligation.

From what area of the spine is the 32 cm. measured? The 32 cm is measured from the bottom of the Lulav [in the area that a leaf begin to grow on each side[2]] until the top of the spine. The top area of the spine where the spine splits into two leaves is not considered part of the spine and thus one must have 32cm of spine from below this area.

Is there a maximum length for the Lulav?[3] No.

 

Summary:

The length of the spine of the Lulav is 32cm. and is to be at least one Tefach higher than the Hadassim and Aravos. It is measured from the bottom of the spine [in the area that a pair of leaves begin to grow on each side[4]] until the top of the spine.

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[1] 650/1

[2] Piskeiy Teshuvos 650/1; Kashrus Daled Minim p. 78

[3] 650/2

[4] Piskeiy Teshuvos 650/1; Kashrus Daled Minim p. 78