By "length" I mean the amount of space on the page in the PDF or printed journal. Obviously the length of the references section will depend on the style, so that will be a factor.
I am thinking of one with a very large number of references.
I am not so interested in a paper where some of the references have very long lists of authors and therefore take up a lot of space, as in the question below which mentions a single reference taking up half a page.
I am also not so interested in a paper that uses a bibliographic references style where not every reference is cited in the text.