Actually, I did not find one.
Nevertheless, I think .length is convenient.
It is not a bug but maybe a wish.

commenting your suggestions:
- size is not the same, it displays two dimensions
- for java arrays, both length(.) and numel(.) are the same
  but this is true only because java arrays are seen as 1 dimensional in octave and nested arrays are not visible.

I attached a patch, you could apply because it improves the implementation, documents further weaknesses
and provides in comments 2 lines fix for 'bug' 48738

(file #38272)

The length of a java array shall be returned using the length attribute:

a=javaArray('java.lang.Object',1,2);a=a(1)

yields (this is a little more complex because of another bug):

a =
<Java object: java.lang.Object[]>

Then we would expect a.length
to return

ans =
2

but obtain
error: [java] java.lang.NoSuchFieldException: length

As a field is expected as for non-array objects.

Suggestion for a bugfix is to add also method is_java_array.