Michael,
Not long ago, I too looked for an Atlantis. I was to have been my third bike from Rivendell. My PBH is 86 cm and my other Rivendell bikes are a 59 cm Homer and a 58 cm Simpleone. Since both of these bicycles fit extremely well, naturally I thought the right size for the Atlantis I wanted would be 59 cm. I learned the hard way that I was wrong.
I wanted a new MIT Atlantis, particularly a 59 cm. Not only did I think it would be similar in size to my other bikes, but I loved the arched second top tube, which was not available on smaller frames. I went ahead and bought a used 59 cm frame from another lister. As it turns out that the sizing of Atlantis models has changed, and I was unaware of this. While I would like have been right with a 59 cm if I was seeking a used Toyo or Waterford Atlantis, my size for the new MIT Atlantis is a 55 cm. The MIT Atlantis is "stretched out" compared to previous models. It has a longer, slanted top tube and longer chain stays. This added length and the slanted top tube means that measuring the size of the bike by the seat tube can be misleading when compared to the earlier Atlantis models that had parallel or nearly parallel top tubes. I was lucky. I learned this before the seller had even shipped me the 59 cm. He was kind enough to refund my payment and had no trouble finding another buyer.
You may be looking for one of the older models. If so, nevermind. If, however, you are looking for an MIT Atlantis, I hope my experience is helpful to you.
Steve Cole
Arlington, Virginia