I would definitely try to make sure it was about art history, and better yet, about something somewhat related to the subfield you want to study (if you want to focus on abstract expressionism, don't submit a paper about byzantine mosaics - unless it's truly exceptional). I would expand on one of your old shorter papers (aim for 15 pages-ish). A lot of people say "they just want to see how you write," which is true, but with admissions being as competitive as they are, you're probably better off demonstrating your skills in your intended field of study.
For what it's worth, during this past application cycle I've learned that you really want to make sure you're putting your best and strongest foot forward. Make sure your writing sample is as strong as possible, that way you won't have any regrets. I submitted a writing sample that was "pretty okay" in my book, and now I'm left wondering if my app would have been stronger had I submitted something better.
Are you applying for MAs or PhDs? I would think that for PhDs you'd want a paper very tailored to what you want to ultimately focus on, but for MAs I'm sure something a little outside of that area (if you even have a chosen area at this point) would be fine.