Все просто.
function LSort(a,b){return a.length-b.length;}
var YourArray=[[1,2,3,4,5,6], ['a','b'], ['X','Y','Z'], ['I','Love','You'], ['good man']];
YourArray.sort(Lsort);
Результат:
['good man'] Length=1
['a','b'] Length=3
['X','Y','Z'] Length=3
['I','Love','You'] Length=3
[1,2,3,4,5,6] Length=6