It's no different than:

  public class LinkedList {
     ...
     public int size() {
        if (head == null) {
           return 0;
        } else {
           int n0 = 1 + head.size();
        }
    }

Actually, it is different. In prolog, you don't tell it how to compute an answer. You tell it what the answer is, and it figures out the rest.

In the previous example, I'd phrase it more like this:

I am sure [] has 0 length. I would be sure [_:Ls] has N length, if I was sure N>0 and N=N0+1 and [Ls] has N0 length.

Notice I never really told prolog how it is supposed to go about figuring out that [Ls] has N0 length -- prolog might use recursion. Or there might be other facts which let it figure out the length of [Ls]. Prolog will try to find a way. For example, I might introduce a rule

list_length(join(a, b), N) :- N #= NA + NB, list_length(a, NA), list_length(b, NB)

Now prolog knows that when you join two lists of known length, the lengths can be summed, without actually counting the length of either list or the combined list. And actually prolog doesn't even know or care what "join" means here, it just knows that "join" will cause the lengths to get summed.