The length of the Hadassim must be at least three Tefachim [24cm.]
How much is three Tefachim? Some Poskim rule that every Tefach is 4 Agudlin and hence three Tefachim is 12 Agudlin [24cm.] Others rules that each Tefach is 3.33 Agudlin and hence three Tefach is 10 Agudlin [20 cm]. Practically we are stringent to follow the first opinion of 24 cm. However in a time of need one may be lenient to use Hadassim of 20 cm. If one already used such a Hadas and then found one with 24 cm. he should shake it without a blessing being that he has already fulfilled his obligation and.
Is there a maximum length for the Hadas? No. The Hadassim may be as long as desired, although one must be careful that the Lulav spine always extends one Tefach above the Hadasim.
 646/3; 650/1