The length of the Aravos must be at least three Tefachim.
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 an Arava of 20 cm. If one already used such a Arava 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 Arava? No. The Aravos may be as long as desired, although one must be careful that the Lulav spine always extends one Tefach above the Aravos.