List l1 = Arrays.asList(1,2,3,4);
List l2 = Arrays.asList(1,2,3,4,5);
List l3 = Arrays.asList(1,2,3,4,5,6);
List<List> lists = Arrays.asList(l1,l2,l3);

lists.stream().reduce(Integer.MIN_VALUE, (a,b) -> Integer.max(a.size(), b.size()));

Type mismatch: cannot convert from int to List

List l1 = Arrays.asList(1,2,3,4);
List l2 = Arrays.asList(1,2,3,4,5);
List l3 = Arrays.asList(1,2,3,4,5,6);
List<List> lists = Arrays.asList(l1,l2,l3);
lists.stream().map(List::size).max(Comparator.naturalOrder()).get();

lists.stream().mapToInt(List::size).max().getAsInt()