JPA Criteria API query - sorting by number of elements in collection

I am having problem with building a query with JPA Criteria API.

Entities and significant properties:

@Entity
public class Post {
@Id int id;
@OneToMany (mappedBy = "post") Set<Comment> comments;
//...

@Entity
public class Comment {
@Id int id;
@ManyToOne Post post;
//...

I need a query that will return all posts from db ordered by number of
comments (`OneToMany` relation in `Post`).
At first I thought this can be implemented with `JPQL` like:

SELECT p
FROM Post p
ORDER BY SIZE(p.comments) DESC

But function `SIZE(...)` can not be used to be ordered by it in `JPQL`.

So, I found about `JPA Criteria API`, and tried following:

CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Post> cq = cb.createQuery(Post.class);
Root<Post> p = cq.from(Post.class);
cq.select(p);
cq.orderBy(cb.desc(p.get("comments")));
List<Post> resultList = em.createQuery(cq).getResultList();

With this query I am not getting proper results. I am aware that I am
missing getting `size` of the set 'comments', but don't know how to add that
part. I am not really familiar with `JPA Criteria API`. How should this
query look to get all posts ordered by size of its `comments` field(set)?